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We present an analysis of the available long-wavelength, high-dispersion spectra of seven hybridchromosphere stars obtained with the International Ultraviolet Explorer between 1978 and 1993. Our investigation of the variability of the Mg n h and k resonance doublet demonstrates that the emissionline fluxes are not rotationally modulated with the periods previously suggested by Brosius, Mullan, & Stencel. Furthermore, we find no evidence in the Mg n data to corroborate the multiple periodicities reported in the Ca n emissions of hybrid stars by Rao et al. Examination of 40 pairs of closely spaced Mg n observations failed to reveal the presence of any strong chromospheric flaring on the sample stars. Significant (20%-40%) nonperiodic Mg n flux variability, on timescales of days to years, is observed in six of the seven stars. The flux variations occasionally are accompanied by dramatic changes in the morphology of the Mg n profiles, indicating variable stellar-wind absorption. We argue that the variability observed is consistent with stochastic changes associated with the slow growth and decay of chromospheric active regions and the gradual evolution of the physical conditions in the winds. It is our opinion that no compelling argument has yet been made for periodic variability in the chromospheric diagnostics of hybrid stars.

We evaluate time series observations of chromospheric lines (Mg u, Mg t, and C n) for the K giant _t Tau obtained using the IUE LWP camera at high dispersion. These observations cover a time span of about 2 weeks in 1994 February March and were designed to resolve variations occurring within hours, days, and weeks. We consider the observational results in relation to theoretical acoustic heating models, motivated by the fact that _t Tau may exhibit a basal (i.e., minimum) level of chromospheric activity. The data reveal flux variations between the extremes of 8% in Mg II h+k and 15% in each emission com-