ON THE DIFFICULTY OF DETERMINING THE COLOR-TERM IN THE CEPHEID PLC RELATION

We present here a new tentative to determine the color-term of the Cep heid period - luminosity - color relation, using a large sample of 100 galactic Cepheids, whose distances are known from the visual surface brightness method (approximately 10% accuracy). The derived coefficien t seems lower (between 0 and 1) than both its observed value in the Ma gellanic Clouds (about 2.1) and the semi-theoretical value of about 3. 4. Systematic effects due to metallicity are investigated, and discard ed as a possible explanation of the discrepancy. Statistical effects a re reviewed, and a formalism useful to other topics is presented. Stat istical corrections remove only part of the discrepancy we find. We th erefore still recommend not to use the PLC relation to determine extra galactic distances, but to prefer a simple PL relation, as long as the question of the PLC color-term is not clearly resolved.