  We present X-ray observations of the close binary nucleus of the planetary
nebula SBS 1150+599A obtained with the XMM-Newton satellite. Only one component
of the binary can be observed in optical-UV. New X-ray observations show that
the previously invisible component is a very hot compact star. This finding
allows us to deduce rough values for the basic parameters of the binary. With a
high probability the total mass of the system exceeds Chandrasekhar limit and
makes the SBS 1150+599A one of the best candidate for a supernova type Ia
progenitor.
