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A Study of the Unusual Z Cam Systems IW Andromedae and V513 Cassiopeia

The Z Camelopardalis stars IW And and V513 Cas are unusual in having outbursts following their standstills, in contrast to the usual Z Cam behavior of quiescence following standstills. In order to gain further understanding of these little-studied systems, we obtained spectra correlated with photometry from the AAVSO throughout a 3–4 month interval in 2011. In addition, time-resolved spectra were obtained in 2012 that provided orbital periods of 3.7 hr for IW And and 5.2 hr for V513 Cas. The photometry of V513 Cas revealed a regular pattern of standstills and outbursts with little time at quiescence, while IW And underwent many excursions from quiescence to outburst to short standstills. The spectra of IW And are similar to normal dwarf novae, with strong Balmer emission at quiescence and absorption at outburst. In contrast, V513 Cas shows a much flatter/redder spectrum near outburst with strong He II emission and prominent emission cores in the Balmer lines. Part of this continuum difference may be due to reddening effects. While our attempts to model the outburst and standstill states of IW And indicate a mass accretion rate near 3 × 10-9 M⊙ yr-1, we could find no obvious reason why these systems behave differently following standstill, compared to normal Z Cam stars.