Eclipse mechanisms for binary pulsars.

The parameters of the new eclipsing millisecond pulsar PSR 1744 - 24A in Terzan 5 are sufficiently different from those of PSR 1957 + 20 that very severe constraints can be put now on the theoretical models and formation scenarios of such systems. Most importantly, the eclipse cannot be caused by refractive effects (either total reflection or bending of the radio signal paths). Instead, they very likely are produced by an absorption mechanism, possibly combined with pulse-smearing effects. A model based on free-free absorption by the gas evaporated from the companion remains viable in spite of difficulties. Independent of the particular eclipse mechanism, the results of some preliminary dynamical calculations of the evaporative flow are discussed.