Supergiant Long-Period Variables (LPVs) are both more luminous and more common than Cepheids. The fact that a P – L relation exists at K (e.g. Wood et al. 1983 ApJ, 272, 99; Mould et al. 1990 ApJ, 349, 503) and not at visible wavelengths is usually attributed to the strong temperature sensitivity of TiO, which dominates the spectra of these stars in the visible. This has led us to apply a narrow bandpass (λ0 = 8250Å, BW = 350Å), avoiding TiO, in the hopes that an optical P – L relation can be found.
The data for Per OB1, the LMC, and M 33 are shown in figure 1. The small dispersion (σLMC
= 0.22 mag.) and flat slope are evident. Combining the data we find:
with the absolute calibration from the ten LPVs in Per OBI, assuming m – M = 11.8±0.2 (Garmany & Stencel 1992 AAS, 94, 211).