Multiwavelength Observations of Three Short‐Period Active Binary Systems: ER Vulpeculae, CC Eridani, and EI Eridani

We present the results of multiwavelength observing campaigns of three short-period active binary systems: ER Vulpeculae, CC Eridani, and EI Eridani, all with orbital periods less than 2 days. ER Vulpeculae (Porb ~ 0.7 day) was observed simultaneously in the X-ray and radio with ASCA and MERLIN, CC Eridani (Porb ~ 1.56 days) with EUVE, the VLA, and the ATCA, and EI Eridani (Porb ~ 1.95 days) with EUVE and ASCA. The ASCA observations of ER Vul and EI Eri reveal a pattern of low coronal metallicities during quiescence, which increase during flares. There is evidence in ASCA spectra of EI Eri for extremely hot, (T > 100 MK) plasma arising during stellar flares. Multifrequency radio emission from CC Eri shows flux decreasing with increasing frequency, and polarization increasing with frequency. During the observations of CC Eri at 20 cm, two highly circularly polarized flares occurred. One, being left circularly polarized at -70%, is consistent with plasma radiation at the fundamental frequency as on other active binary systems. The second flare is largely right circularly polarized (up to 100%). The results from EUVE observations suggest high coronal electron densities ne > 1012 cm-3, which is in conflict with the implied electron density of the radio-emitting plasma if the two types of emission have a common electron population. Despite our long EUV and X-ray observations of these systems lasting several orbital periods, flaring at these wavelengths was observed only on EI Eri, the longest period system here. We find a very peaked distribution of count rates about the average for all the nonflaring high-energy observations.