A search for giant and asymptotic-giant-branch variable stars in six globular clusters

Photographic photometry has been obtained via a PDS microdensitometer in a systematic search for variability of stars on the giant and asymptotic-giant branches of six globular clusters. Typical rms measurement errors of σB ≡ 0.04 mag for the brighter stars permit detection of variability with an amplitude AB ⪆ 0.2 mag. Within that limit, the author found variables only near the tip of the giant branch; most stars with Mbol ⪉ -3.0 probably are small-amplitude variables. Variability for any fainter giant and asymptotic-giant-branch stars is not confirmed. Lack of further information (reliable periods, variations in color and spectra) for most of the variables precludes detailed determination of their properties; possible correlations with Hα emission and polarization are briefly noted.