Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method .
Résumé
We derive individual distances to six Cepheids in the young populous star cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface brightness technique. With six stars available at the exact same distance we can directly measure the intrinsic uncertainty of the method. We find a standard deviation of 0.11 mag, two to three times larger than the error estimates and more in line with the estimates from Bayesian statistical analysis by Barnes et al. \cite{Barnes05}. Using all six distance estimates we determine an unweighted mean cluster distance of 18.30±0.05. The observations indicate that NGC1866 is close to be at the same distance as the main body of the LMC. If we use the stronger dependence of the p-factor on the period as suggested by Gieren et al. \cite{Gieren05} we find a distance of 18.50±0.05 (internal error) and the PL relations for Galactic and MC Cepheids are in very good agreement.