The numbers of luminous SSSs in the disks of ordinary spiral galaxies such as the MW and M31 are estimated to be on the order of 103. SSXSs have now been discovered in our galaxy and in globular cluster M3. MR Velorum (RX J0925.7-4758) is one of the rare MW super soft X-ray binaries. "The source is heavily reddened by interstellar material, making it difficult to observe in the blue and ultraviolet." The period determined for MR Velorum at ~4.03 d is considerably longer than that of other supersoft systems, which is usually less than a day. The CBSS model invokes steady nuclear burning on the surface of an accreting white dwarf (WD) as the generator of the prodigious super soft X-ray flux. As of 1999, eight SSXSs have orbital periods between ~4 hr and 1.35 d: RX J0019.8+2156 (MW), RX J0439.8-6809 (MW halo near LMC), RX J0513.9-6951 (LMC), RX J0527.8-6954 (LMC), RX J0537.7-7034 (LMC), CAL 83 (LMC), CAL 87 LMC), and 1E 0035.4-7230 (SMC). A symbiotic binary star is a variable binary star system in which a red giant has expanded its outer envelope and is shedding mass quickly, and another hot star (often a white dwarf) is ionizing the gas. Three symbiotic binaries as of 1999 are SSXSs: AG Dra (BB, MW), RR Tel (WD, MW), and RX J0048.4-7332 (WD, SMC). The youngest, hottest WD, KPD 0005+5106, is very close to 100,000 K, of type DO and is the first single WD recorded as an X-ray source with ROSAT. "Cataclysmic variables (CVs) are close binary systems consisting of a white dwarf and a red-dwarf secondary transferring matter via the Roche lobe overflow." Both fusion- and accretion-powered cataclysmic variables have been observed to be X-ray sources. The accretion disk may be prone to instability leading to dwarf nova outbursts: a portion of the disk material falls onto the white dwarf, the cataclysmic outbursts occur when the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion reactions, which rapidly burn the hydrogen layer to helium.