TESTING THE LENSING HYPOTHESIS FOR COMPACT DOUBLE RADIO-SOURCES AND OTHER MILLI-LENSING CANDIDATES

Condensed objects of approximate to 10(6) M. could manifest themselves via their lensing action on the radio cores of background quasars, th us giving rise to compact double sources, or more complex small-separa tion lenses, on the milli-arcsecond scale. Such compact bodies could b e dense cores of small galaxies or remnants of pregalactic stars which have been postulated as a candidate for the dark matter in the univer se. Search for examples of milli-arcsecond scale lensing, either due t o such remnants or dwarf galaxies, forms a central objective of severa l new VLBI projects which has already revealed a good candidate. Here we point out that the milli-lensing of superluminal cores of quasars/b lazars, when it occurs, can lead to substantial changes in the apparen t orientation, flux ratio and velocities of the image components over periods as short as a year. This is essentially because the proper mot ion of the sub-components of such cores is comparable to the image spl itting scales. Hence VLBI monitoring programmes can provide a fairly s imple test of the milli-lensing hypothesis.