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Close binary systems with orbital periods from 0.05 to 230 days. One of the components of these systems is a hot dwarf star that suddenly, during a time interval from one to several dozen or several hundred days, increases its brightness by 7-19 mag in V, then returns gradually to its former brightness over several months, years, or decades. Cool components may be giants, subgiants, or dwarfs of K-M type (from GCVS).

  • there is roughly 30 novae per year in the Milky Way and about 20 in other galaxies
  • absolute magnitude <math>M_V = -8 mag</math>
  • spectra near maximum light resemble A-F absorption spectra of luminous stars at first
  • spectra at minimum light resemble spectra of Wolf-Rayet stars.
  • amplitude between 7 and 19 mag in V
  • quick rise of few days
  • slow decline, between few days and 100 days (fast: type A, slow: type B, very slow: type C)
  • 15 days after maximum its Mv = -5.5mag, independent on speed or class
  • small changes at minimum light may be present

Recurrent novae:

  • are produced by a white dwarf star and a red giant circling about each other in a close orbit
  • recurrent every 10-100 years
  • smaller amplitude: 4-9 mag
  • M_V at minimum is about 0-1 mag
  • example: RS Oph - 3 outbursts between 1957 and 1991 (AAVSO), exhibits some small variation in the baseline in minimum
  • decline within few days

Short transient searches in the other galaxies revealed possible new sub-class of novae: Kasliwal et al. 2010. They usually stay above the Gaia detection limit for not more than 10 days.