RS Ophiuchi is a recurrent nova,
which is a variable star and a binary star,
consisting of a white dwarf and red giant.
It is one of just a very few observed recurrent novae in
the Milky Way and has the largest number of recorded eruptions,
seven, the first in 1898 and the most recent in 2021.
Its frequent recurrence and relative nearness make it
of considerable research interest.
Characteristics:
WD and K5.5/M0IIIe - spectral types (the giant star's type varies between M and K even when the nova is quiet).
about +12.5 - apparent magnitude when quiet, growing to less than +5 during an eruption.
2600 pc - distance (note that determinations have been inconsistent and are not fully trusted).
Other such Milky-Way recurrent novae include
T Pyxidis (T Pyx) which erupted in 1944, 1967, and 2011,
T Coronae Borealis (T CrB) which erupted in 1866 and 1946,
and
IM Normae (IM Nor) which erupted in 1920 and 2002.