  We have analysed 350 pointed and serendipitous observations of 108 different
classical and recurrent novae in outburst and in quiescence, contained in the
ROSAT archive. One aim was to search for super-soft X-ray sources and we found
only 3 of them among post-novae. Thus, the super-soft X-ray phase of novae is
relatively short lived (up to 10 years) and is observed only for up to 20% of
novae.
  Most classical and recurrent novae instead emit hard X-rays (in the ROSAT
band) in the first months after the outburst, with peak X-ray luminosity of a
few times 10(33) erg/s. The emission, which we attribute to shocks in the nova
ejecta, lasts at least 2 years and even much longer under special circumstances
(like preexisting circumstellar material, or a prolonged wind phase).
  We also investigate X-ray emission due to accretion in quiescent novae. Only
11 out of 81 Galactic classical and recurrent novae were detected. The average
X-ray uminosity is not higher than for dwarf novae, and some novae are variable
in X-rays on time scales of years.
