2002A&A...384..460B

A multicolour photometric data basis is compiled for SU Dra from 356 Johnson UBV and 228 UBV(RI)_C observations reported in this paper, in addition to 483 BV, 289 UBV, and 85 UBVRI observations found in the literature. Mean period, period change (4.40±.14)x10^–10day/day, and phase noise are derived from the V light curves by a variational procedure minimizing the length of the folded light curve (string length minimization, SLM). From variation of string length in the neighbourhood of minimum, observational error of photometry, observed amplitude, number of observations formulae are derived for the error of period and phase noise by statistical and analytical considerations. Secondary periodic variations are excluded to a few mmag level, indication of amplitude variation (Blazhko-effect) has not been found, however, systematic segregation of light curve segments is reported from different epochs in the descending branch before the brightness rise. Mean light and colour curves are derived from photometry and they are compared with those from integration of spectrophotometric observations. The period change is interpreted in terms of mixing events in the stellar interior.