Short time scale variability in Mkn 501

In the TeV blazars Mkn421 and Mkn501, the same electrons are thought to be responsible for the synchrotron X-rays and TeV Comptonized emission. As a p relude to simultaneous X-ray/TeV analysis, we present results from variabil ity studies of Mkn 421 and Mkn 501. We find that each source has a variabil ity of 5% r.m.s. amplitude on time scales shorter than 20 minutes. We find some evidence that the hard X-ray flux may be more variable than the soft X -ray flux in Mkn 421. For Mkn 501 we also present comparisons with nearly s imultaneous TeV observations taken by the HEGRA collaboration. (C) 1999 Els evier Science B.V. All rights reserved.