In nova-like stars the binary system is visible.
These stars exhibit "only irregular small-scale brightness changes or occasional drops in luminosity".
They have accretion disks.
"There exist two sub-classes of nova-like stars, the DQ Herculis stars and the AM Herculis stars, whose white dwarfs possess magnetic fields of appreciable strength which dominate the accretion disk and basically all phenomena related to it."
DQ Herculis stars
The "DQ Herculis stars [are] cataclysmic variables containing an accreting, magnetic, rapidly rotating white dwarf. These stars are characterized by strong X-ray emission, high-excitation spectra, and very stable optical and X-ray pulsations in their light curves."
"The white dwarfs' magnetic moments are in the range 1032-1034 G cm3, slightly weaker than in AM Her stars but with some probable overlap."
"DQ Hers have broken synchronism [which] is probably [due to] their greater accretion rate and orbital separation."
"X-ray emission from short-period systems appears to be weaker and softer."
Studying "the light curve of the remnant of Nova Herculis 1934 (=DQ Herculis), Merle Walker found strictly periodic variations with the amazingly short period of 71 s (Walker 1954, 1956)."
X-ray source: 2RXP J180730.0+455136
SIMBAD Query : otype='DQ*' lists 49 *s.
"There are many cataclysmic variables for which there is [...] an increasing number of objects in which magnetic fields do appear to play a role which [...] is sufficient to introduce the classification "intermediate polar"."
Characteristics include "an X-ray beam emitted from the [slowly] rotating [but asynchronous] degenerate star [which] illuminates either the companion [...] or the gas in the extended hot spot region [...]. Neither of the stars show optical polarization indicating magnetic fields at least an order of magnitude lower than in the polars."
"No positive detection of circular polarization has yet been made in the intermediate polars."
"In the polars, circular polarization is attributed to cyclotron emission from the accretion column [...]."
X-ray sources: CXOU J002257.6+614107, PBC J0023.0+6138, 1RXS J002258.3+614111, SWIFT J0023.2+6142, 2XMM J002257.7+614107
X-ray source: PBC J0028.9+5917, RX J0028.8+5917, 1RXS J002848.2+591723, SWIFT J0028.9+5917, SWIFT J0028.6+5918
X-ray source: 1AXG J025609+1926, 2E 677, 2E 0253.3+1914, H0253+193, PBC J0256.1+1925, SWIFT J0256.2+1925, 2XMM J025608.1+192634, XSS J02569+1931
X-ray source: 1A 0327+43, 3A 0327+438, 2E 785, 2E 0327.7+4344, PBC J0331.1+4353, 1RXS J033111.9+435427, SWIFT J0331.1+4355, SWIFT J0331.2+4354.
"Using NASA’s Chandra X-ray Observatory, astronomers [...] pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness."
"GK Persei [is] an example of a “classical nova,” an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star."
"Chandra first observed GK Persei in February 2000 and then again in November 2013. This 13-year baseline provides astronomers with enough time to notice important differences in the X-ray emission and its properties."
"This new image [on the right] of GK Persei contains X-rays from Chandra (blue), optical data from NASA’s Hubble Space Telescope (yellow), and radio data from the National Science Foundation’s Very Large Array (pink). The X-ray data show hot gas and the radio data show emission from electrons that have been accelerated to high energies by the nova shock wave. The optical data reveal clumps of material that were ejected in the explosion. The nature of the point-like source on the lower left is unknown."
"The X-ray luminosity of the GK Persei remnant decreased by about 40% over the 13 years between the Chandra observations, whereas the temperature of the gas in the remnant has essentially remained constant, at about one million degrees Celsius. As the shock wave expanded and heated an increasing amount of matter, the temperature behind the wave of energy should have decreased. The observed fading and constant temperature suggests that the wave of energy has swept up a negligible amount of gas in the environment around the star over the past 13 years. This suggests that the wave must currently be expanding into a region of much lower density than before, giving clues to stellar neighborhood in which GK Persei resides."
X-ray source: 2E 1206, 2E 0459.4+2441, 1H 0459+248, H0459+246, H 0500+24, PBC J0502.4+2443, SWIFT J0502.4+2446, SWIFT J0502.7+2445, XSS J05019+2444
X-ray source: RX J0512.2-3241, 1RXS J051214.5-324140
X-ray sources: 2A0526-328, 3A 0527-329, 1E 0527.5-3251, 2E 1286, 2E 0527.5-3251, A0526-328, 1ES 0527-32.8, 1H 0527-328, PBC J0529.3-3249, 1RXS J052925.8-324858, SWIFT J0529.2-3247, XSS J05295-3252
X-ray sources: 1AXG J055800+5353, 2MAXI J0558+540, PBC J0558.0+5353, RX J0558.0+5353, 1RXS J055800.7+535358, SWIFT J0558.0+5352, SWIFT J0557.8+5353
X-ray sources: PBC J0625.2+7336, 1RXS J062518.2+733433, SWIFT J0625.1+7336
BG Canis Minoris
The "observations of BG Canis Minoris revealed circular polarization, strongly suggesting the presence of a ~4 x 106 G field (Penning et al. 1986; West et al. 1987). Polarization has also been observed in RE 0751+144 (Pürola et al. 1993)."
X-ray source: 3A0729+103, 2E 1822, 2E 0728.7+1002, 2MAXI J0730+100, PBC J0731.5+0955, SWIFT J0731.5+0957, SWIFT J0731.4+0954
X-ray source: PBC J0732.6-1331, 1RXS J073237.6-133113, SWIFT J073237.6-133109, Swift J0732.5-1331, SWIFT J0732.6-1330
X-ray source: 1AXG J075117+1444, 2MAXI J0751+149, PBC J0751.2+1445, 2RE J075119+144443, 2RE J0751+144, RE0751+14, RE J0751+144, RE J075120+144510, RX J0751.2+1444, SWIFT J0750.9+1439, SWIFT J0751.1+1442, 2XMM J075117.4+144425, XSS J07514+1442
X-ray source: RX J0757.0+6305, RX J0757.0+6306, 1RXS J075700.5+630602, XSS J08010+6241, [ZEH2003] RX J0757.0+6306 1
Accordng to SIMBAD, "According to the GCVS team, YY Dra is considered to be another star. Whereas DO Dra is a well-documented dwarf nova, YY Dra is a lost eclipsing binary, which original coordinates are probably erroneous (see discussion in 1987IBVS.3079....1P and 1988IBVS.3154....1K)".
X-ray source: 2A 1150+720, 3A1148+719, 1E 1140.8+7158, 1E 1140.7+7158, 2E 2515, 2E 1140.7+7158, 1ES 1140+71.9, 2MAXI J1144+717, PBC J1143.6+7141, RX J1143.6+7141, 1RXS J114338.6+714125, SWIFT J1142.7+7149, XSS J11474+7143, [ZEH2003] RX J1143.6+7141 1.
X-ray source: 2MAXI J1237-387, PBC J1238.1-3843, RX J1238-38, RX J1238.2-3842, 1RXS J123816.5-384243, SWIFT J1238.1-3842, XSS J12392-3820.
X-ray source: 2A 1251-290, 3A 1250-289, 2E 2876, 2E 1249.7-2858, 1ES 1249-28.9, EXO 1249.7-2858, 1H 1251-291, 1M 1252-289, PBC J1252.3-2914, 2RE J125223-291506, 2RE J1252-291, RE J125223-291445, RE J1252-291, RX J1252.4-2914, 1RXS J125224.7-291451, SWIFT J1252.3-2916, 2U 1253-28, 3U 1252-28, 4U1228-29, 4U 1249-28, 2XMM J125224.2-291456, XSS J12529-2911.
X-ray source: PBC J1548.1-4528, 1RXS J154814.5-452845, SWIFT J1548.0-4529, SWIFT J1548.2-4529, 2XMM J154814.4-452839, [KRL2007b] 173.
X-ray source: PBC J1712.6-2415, RX J1712.6-2414, 1RXS J171236.3-241445, SWIFT J1712.7-2412, SWIFT J1712.7-2417, 2XMM J171236.3-241445, [KRL2007b] 230.
X-ray sources: 3A 1851-312, 1ES 1851-31.2, 1H 1853-312, 2MAXI J1854-310, PBC J1854.9-3109, 1RXS J185502.1-310951, SWIFT J1855.0-3110, SWIFT J1854.9-3109, 4U 1849-31, 4U 1851-31, XSS J18553-3111, [KRL2007b] 344.
X-ray source: PBC J1958.2+3232, SWIFT J1958.3+3233, 1WGA J1958.2+3232.
X-ray source: 1AXG J204011-0052, 1E 2037.5-0102, 2E 4404, 2E 2037.5-0102, 1ES 2037-01.0, 1RXS J204009.4-005216, 2XMM J204009.0-005214.
X-ray source: RX J2133.7+5107, 0ES 2132+50.9, PBC J2133.9+5106, 1RXS J213344.1+510725, SWIFT J2133.6+5105, SWIFT J2133.6+5107, 2XMM J213343.5+510723, [KRL2007b] 387.
X-ray source: 2E 4588, 2E 2215.3-0835, H 2215-086, 2MAXI J2218-083, PBC J2217.9-0820, 1RXS J221753.9-082115, SWIFT J2217.5-0812, 2XMM J221755.3-082103, XSS J22178-0822, [KRL2007b] 394.
X-ray source: 3A 2253-033, 1AXG J225518-0310, 2E 4648, 2E 2252.7-0326, 1H 2251-035, H2252-035, H 2254-033, PBC J2255.3-0310, 1RXS J225518.1-031040, SWIFT J2255.4-0309, XSS J22551-0309.
AM Herculis stars
In "the AM Herculis stars, the magnetic field of the white dwarf prevents the formation of an accretion disk.
The "AM Herculis stars [are] additionally characterized by spin-orbit synchronism and the presence of strong circular polarization."
SIMBAD Query : otype='AM*' lists 95*s.
"The term cataclysmic variable [..] comprises several related types of objects. For one there are the so-called novae, objects whose brightness has changed by ten to twenty magnitudes once in historical times, or recurrent novae whose amplitudes are on the small side but which have been seen to erupt more often than once; furthermore, dwarf novae whose brightness keeps changing by three to five magnitudes in semi-periodic intervals of time of some ten to one hundred days; and finally nova-like stars, which do not undergo outbursts but only irregular small-scale brightness changes or occasional drops in luminosity, but which in all other aspects are similar to the former group."
In "dwarf novae and nova-like stars the binary system itself is visible, [with] processes which can be traced back directly to the presence of an accretion disk in these systems."
The "primary component of which is a white dwarf."
"The secondary components of cataclysmic variables are cool main sequence stars of spectral type approximately solar of later. Such stars are known to possess fairly active surfaces having large star spots associated with appreciable magnetic activity. Even in single stars the physical structure of such an atmosphere is not well understood, and a consistent theory is still to be developed."
A nova-like star is a close binary system with a white dwarf as a primary and a "late-type main-sequence secondary" star filling its Roche lobe. "The secondary loses mass through the inner Lagrangian point and in order to conserve angular momentum the transferred material usually forms an accretion disk around the white dwarf component. A hot spot originates at the place where the mass-transfer stream impacts the disk." In these star systems the degree of magnetic fields ranges from non-magnetic to highly magnetic. "For systems in which the primaries have strong magnetic fields, the process of forming the accretion disk is disturbed. The transferred material is forced to follow the field lines and creates accretion columns near one or both of the white dwarfs magnetic poles."
"The shortest orbital periods imply typical dimensions for the systems to be of the order of a solar diameter."
UX Ursae Majoris
"Spectroscopic investigations (Walker and Herbig (1954)) [...] were of sufficiently good quality for the determination of radial velocities [and included] photoelectric measurements [for UX UMa.] But in 1954 the orbital light curve of the old nova DQ Her was solved, and its similarity to that of UX UMa became obvious; its orbital period is shorter by only 5 minutes, and the shape of the eclipse and the hump near to it make the two so similar to one another that one can easily be mistaken for the other (Walker 1954 and 1956)."
As far as long-term brightness changes are concerned, magnetic and non-magnetic nova-like stars behave in the same way. That the central star in some systems has magnetic properties has nothing to do with the outburst behavior.
The evolution of non-magnetic dwarf novae and nova-like stars can be different from the magnetic systems (polars and intermediate polars). Magnetic and non-magnetic systems display different kinematical properties since some flow velocities come from magnetically channeled plasma.
Non-magnetic systems appear to be much more prevalent than magnetic ones, although the number of magnetic systems is small and near the limit of statistical significance when compared to the non-magnetic systems.
X-ray source: J133640.9+515450.
According to SIMBAD UX Ursae Majoris is a Nova-like Star, Query : otype='NL*' lists 115 *s.
"Gas around V838 Monocerotis, nova-like star, seemed to be expanding faster than light from the earth."
"This image [on the right] shows a time sequence of Hubble Space Telescope images of the light echo around V838 Mon, taken between May 2002 and [February] 2004. All [five] pictures were taken with Hubble's Advanced Camera for Surveys using filters sensitive to blue, visible, and infrared wavelengths. The apparent expansion of the light echo, as light from the early 2002 outburst of V838 Mon propagates outward into the surrounding dust".
"All of the images are shown at the same scale. Moreover, the images are also shown as they would appear for the same exposure times throughout the sequence. Thus the background stars appear constant in brightness, while the surface brightness of the light echo steadily declines. The fading of the light echo is primarily due to the light-scattering properties of interstellar dust. Consider a street lamp on a foggy night. The halo around the lamp is brightest right next to the lamp, while out to the side it is much fainter. Similarly, in the first V838 Mon image, taken in May 2002, the light echo was very bright and compact. At later times, we are seeing dust out to the side of the star, rather than dust that is immediately in front of the star, so the amount of light scattered in our direction is smaller. Hubble astronomers expect the light echo to continue to change its appearance and brightness over the next several years."
- C. La Dous (March 1994). "Observations and Theory of Cataclysmic Variables: On Progress and Problems in Understanding Dwarf Novae and Nova-Like Stars". Space Science Reviews 67 (1-2): 1-221. doi:10.1007/BF00750527. http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994SSRv...67....1L&link_type=ARTICLE&db_key=AST&high=54d6be0a4424362. Retrieved 2016-09-29.
- Joseph Patterson (March 1994). "The DQ Herculis Stars". Publications of the Astronomical Society of the Pacific 106 (697): 209-38. doi:10.1086/133375. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1994PASP..106..209P. Retrieved 2016-10-08.
- Brian Warner (1983). Mario Livio and Giora Shaviv (ed.). The Intermediate Polars, In: Cataclysmic Variables and Related Objects. Netherlands: Springer. pp. 155–172. doi:10.1007/978-94-009-7118-9_20. ISBN 978-94-009-7120-2. Retrieved 2016-10-10.
- Jennifer Harbaugh (13 March 2015). ""Mini Supernova" Explosion Could Have Big Impact". Washinton, DC USA: NASA. Retrieved 2016-10-12.
- Danuta Dobrzycka and Steve B. Howell (April 1992). "Spectroscopic observations of the cataclysmic variable PG 0917 + 342 - an ultra short-period nova like system". The Astrophysical Journal 388 (4): 614-20. doi:10.1086/171178. http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992ApJ...388..614D&link_type=ARTICLE&db_key=AST&high=54d6be0a4412709. Retrieved 2016-09-30.
- VP Kozhevnikov, PE Zakharova, and TP Nikiforova (January 2004). "Short-term brightness variations of V747 Cyg". New Astronomy 9 (1): 51-7. doi:10.1016/S1384-1076(03)00086-1. http://adsabs.harvard.edu/abs/2004NewA....9...51K. Retrieved 2016-09-30.
- T Ak T, S Bilir, S Ak, KB Coskunoglu, and Z Eker (August 2010). "The age of cataclysmic variables: a kinematic study". New Astronomy 15 (6): 491-508. doi:10.1016/j.newast.2009.11.007. http://adsabs.harvard.edu/abs/2010NewA...15..491A. Retrieved 2016-09-30.
- Akira Inaka (2 October 2015). V838 Monoceroti. Baltimore, Maryland USA: Space Telescope. Retrieved 2015-10-07.
- Z. Levay (3 February 2005). Light Continues to Echo Three Years After Stellar Outburst. Baltimore, Maryland USA: Hubblesite. Retrieved 2015-10-07.