SS Cygni is the brightest U Geminorum star (dwarf nova) and the second to be discovered (after U Gem itself in 1855), by Louisa D. Wells of the Harvard College Observatory in 1896. It is the type object for the SS Cygni star subclass and one of the most observed variable stars in the sky. SS Cyg lies about 90 light-years away in the constellation Cygnus and consists of an orange-yellow dwarf of mass 0.60 solar mass and a white dwarf of mass 0.40 solar mass orbiting around each other in 6 hours 38 minutes at distance (surface to surface) of 160,000 km or less.
Three-quarters of the time, SS Cyg is quiescent with a visual magnitude of 12.2; then it begins to brighten without warning and reaches peak brightness of 8.3 in a day or so. Outbursts last 1 to 2 weeks and are repeated every 4 to 10 weeks with a mean time between eruptions of 54 days. A closer look at SS Cyg's light curve reveals ever-changing intervals of long (L) and short (S) outbursts, of about 18 and 8-day durations, respectively. In addition, there are occasional anomalous outbursts, wide and symmetrical in shape, which occur with a slow rise. Although the star has typically displayed this changing outburst characteristic since its discovery, it went through a period from 1907 to 1908 when it abandoned its normal outburst behavior and only underwent only minor fluctuations. A statistical study of SS Cygni's light curve revealed that the most common sequence of outbursts is LS (with 134 occurrences), followed by LLS (69), LSSS (14), and LLSS (8). Together these strings represented 89% of the outbursts studied. According to one suggestion, what determines whether an outburst will be a long or short is the amount of mass in the disk at the start of a thermal instability: a short outburst corresponding to moderate mass transfer, while a long outburst results from a major mass transfer.