HIDDEN MILLISECOND PULSARS

The eclipse characteristics of the two currently known millisecond pulsars vaporizing their companion stars (PSR 1957 + 20 and PSR 1744-20A) are sufficiently different to allow a classification in three "types" of "star-vaporizing pulsars" (hereafter SVPs). PSR 1957 + 20 is the prototype of SVPs with stable eclipse properties (type I) whereas PSR 1744-24A shows a more erratic and sometimes intermittent behavior of radio-eclipses characteristics of a second class of SVPs (type II). In addition to these two SVP classes, a third class of SVPs is expected on theoretical grounds (type III). The SVPs of type III are completely engulfed by the evaporated material from the irradiated companion star, and they are therefore "hidden" pulsars because their radio-emission may be completely blocked. A quasi-steady state is possible for type III SVPs and pulsar radiation pressure and the centrifugal barrier may prevent accretion on the rapidly spinning neutron star if the companion star is underfilling its Roche lobe. The relevance of this class of binary pulsars for ROSAT and GRO observations is briefly discussed together with the possible resolution of the apparent discrepancy between the orbital period distribution of binary millisecond pulsars versus their LMXB progenitors.