Volume 25 number 2 (1997)
The spectra of Mira variables are extremely complex, with atomic lines in absorption and emission and strong molecular bands, all varying with phase. Magnitudes measured by eye or with wide-band photometry represent averages over so much information that they are hard to interpret. Here narrow-band photometry can help, since filters can be chosen both to avoid absorption bands and to measure some of the strongest ones. Some surprising results that have been obtained for Miras by this method are discussed. A simple three-color system measuring two near-infrared continuum points and a TiO (titanium oxide) band can be use to obtain a magnitude, a color index, and a spectral type for normal M-type Miras. The light curves obtained with such a system can be interpreted in terms of variations in luminosity, temperature, and diameter.