X-ray emission of Nova Puppis 1991: Accretion or a shocked shell?

Nova Puppis 1991 (V351 Pup) was observed and detected in X-rays by the ROSAT Position Sensitive Proportional Counter 16 months after the visual maximum with a count rate of 0.223 +/- 0.005 counts s(-1). With follow-up optical observations we determined the value E(B-V) = 0.3 +/- 0.1 for the interstellar absorption and D = 4.7 +/- 0.6 kpc as the distance to the nova. The best spectral fit to the X-ray data is a model of thermal plasma at temperature kT similar or equal to 1.1 keV and source flux f(x) similar or equal to 3 x 10(-12) ergs cm(-2) s(-1) in the range 0.2-2.4 keV, implying an X-ray source luminosity of 7.5 x 10(33) ergs s(-1). The source brightness varied between the two ROS;IT exposures taken 2 days apart. The X-ray emission could be due to restored accretion at a very high rate or to shocks in the ejected shell; it does not have the characteristics of a ''supersoft'' thermal X-ray source associated with nuclear burning on a white dwarf. Nova Puppis 1991 is the third classical nova that emits hard X-rays, among five that have been observed by ROSAT during outburst.