Uuher Uu Herculis stars

Observation

^r Bright stars

^ Mixed amplitudes r\ Mixed periods

- These stars are not recognized within the GCVS but have been the subject of study since 1928. UU Her stars are sometimes described as a subset of the semiregular D variables (SRD). The prototype star UU Her has been the subject of interest since its remarkable behavior was noticed in 1928. The suggestion has been made that this star switches between fundamental and first overtone modes.

As evidence, in 1899 and 1900 this star had a period close to 45 days, with an amplitude of about 1™5. In 1901 the period changed to a much longer -72 days, with an amplitude of 0n.'8. This period persisted until 1905 when the star's period suddenly changed back to 45 days. This period remained stable until 1910 when the amplitude become so small that the star was labeled "invariable." Subsequent data from 1961 indicated a period of 45.6 days, and in 1984 a period of 71.6 days was indicated. Based upon the available information, it appears that UU Her switches periods between -45 days and -72 days.

In recent literature, three stars have been suggested as being classic members of this group: UU Her, 89 Her (V441 Her) and HD 161796. UU Her has often been listed among the RV Tauri stars but appears to now be listed as an SRD star. As you know, the hallmark of the RV Tauri stars is the alternating deep and shallow minima with a difference in depth being typically 0™5-lm. For UU Her, this phenomenon is present at a level of a few hundredths of a magnitude.

The star 89 Her shows bizarre behavior too. In 1977, a regular pulsation cycle with a period of -64 days was evident. In 1978, the photometric pulsation suddenly changed to random fluttering that persisted through 1979. The regular pulsation cycle was reestablished in 1980.

When studied in 1983, HD 161796 seemed to possess two well-defined periods of 62 and 43 days, followed by an interval of non-variability. It was suggested that these two periods represent the fundamental and first overtone of radial pulsation. A study conducted in 1984 indicates that the amplitude of this star increased by about 50% and the star seemed to have brightened. Analysis of the data indicted a period of ~38 days with a second set of data indicating a period of -54 days.

These stars are currently not listed as UU Her stars as this classification is provisional at best. Perhaps as more data is collected, a better understanding of these stars will develop.

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