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ER Vul is a detached binary consisting of a pair of rapidly rotating (P = 0.69d) solar-like G1 and G2 V stars with the strongest coronal and chromospheric emissions of any main-sequence G-type star. This activity most likely arises from a vigorous magnetic dynamo produced by tidally enforced rotation. Consequently, it is important to study ER Vul because the physical characteristics of the components (temperatures, masses, and radii) are nearly identical to the Sun except that the stars rotate over 40 times faster. In addition, it is a binary that may be evolving toward the contact state due to large angular momentum loss through magnetic braking and could therefore play a significant role in understanding the formation of W UMa systems. ER Vul can be a unique laboratory for studying solar-like activity at extreme levels for stars that, while rotating rapidly, are not in the complicating contact configuration.
Observations of ER Vul were carried out in September 1991 for more than two of its 16 hour orbits with the \IUE\ satellite and with ground-based photometry. UV continuum light curves from 1750 to 3150 \AA\ have been produced. Spectroscopically, emission line fluxes of C \II, C \IV, He \II\ and others show orbital phase correlations with the asymmetric continuum light curves. For example, the strongest C \IV\ emission occurs during the orbital phases when the UV and visual continuum fluxes are greatest. We will discuss various models that attempt to explain the behavior of this active binary system.
We gratefully acknowledge the support of NASA grant NAG 5-382 for the \IUE\ program. We also thank NSF for grant AST-8616362 in support of the Four College Consortium.
