The unique system HD49798-RXJ0648 a post spiral-in and common-envelope binary

HD49798RXJ0648 is a unique spectroscopic binary containing a 13 s Xraypulsating star accreting from an early type subdwarf. Although predicted byevolutionary models of common envelope spiralin, no other system of this kindis currently known. The next important step, objective of this proposal, is todetermine if the compact star is a neutron star or, as it seems more likely, awhite dwarf. In the latter case the 13 s spin period would be the shortest knownfor a white dwarf, implying a large accreted mass and making this system thewhite dwarf analogous of LMXRBs containing millisecond pulsars. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]