The angular diameter and distance of the Cepheid zeta Geminorum

Cepheids are the primary distance indicators for extragalactic astronomy an d therefore are of very high astrophysical interest. Unfortunately, they ar e rare stars, situated very far from Earth. Though they are supergiants, th eir typical angular diameter is only a few milliareseconds, making them ver y challenging targets even for long-baseline interferometers. We report obs ervations that were obtained in the K' band (2-2.3 mum), on the Cepheid zet a Geminorum with the FLUOR beam combiner, installed at the IOTA interferome ter. The mean uniform disk angular diameter was measured to be 1.64 +/-0.14 0.16 mas. Pulsational variations are not detected at a significant statist ical level, but future observations with longer baselines should allow a mu ch better estimation of their amplitude. The distance to zeta Gem is evalua ted using Baade-Wesselink diameter determinations, giving a distance of 502 +/- 88 pc.