2.3.2. Metallicity Effects in the LMC Cepheid Calibration
The Cepheid distance to LMC is based on the calibration using open cluster Cepheids, the distances to which are estimated by B star main sequence fitting that ties to the Pleiades (Sandage & Tammann 1968, Caldwell, 1983, Feast & Walker 1987, Laney & Stobie 1994). Metallicity has been measured for some of these calibrator Cepheids (Fry & Carney 1997). The residual of the PL fit shows a strong metallicity (Z) dependence. This means either the Cepheid PL relation suffers from a large Z effect, or the distances to open clusters contain significant Z-dependent errors (Sekiguchi & Fukugita 1998). A correction for this effect changes the distance to LMC in either way, depending upon which interpretation is correct.
This metallicity dependence problem can be avoided if parallaxes are used to find the distances to calibrator Cepheids. Attempts were made (Feast & Catchpole 1997; Luri et al. 1998; Madore & Freedman 1998) using field Cepheids in the Hipparcos catalogue. Unfortunately, Cepheid parallax data are so noisy (only 6 have errors less than 30%) that they do not allow calibrations tighter than ladders. Another skepticism is that 2/3 of Cepheids in the nearby sample (e.g., 14/26 in the Feast-Catchpole sample) are known to have companion stars, which would disturb the parallax (Szabados 1997).