2 Observations

We obtained B and V high state photometry of AMHer in August 1998 at the Loiano 1.5m telescope using a 2-channel photoelectric photometer and a time resolution of 1s (Table 1). The observations were interrupted every 60-120 min for sky measurements in both channels. Data reduction included sky subtraction, differential extinction, absolute calibration using Landolt standards, and binning into 5s bins. The timings have been converted to orbital phases using the ephemeris of Heise & Verbunt (1988), but defining $\phi\,(=\mbox{$\phi_{\rm orb}$ })=0.0$ as the inferior conjunction of the secondary star (Southwell et al. 1995). In AMHer, the conversion between the phase conventions is given by $\mbox{$\phi_{\rm orb}$ }=\mbox{$\phi_{\rm mag}$ }+0.367$. The high precision of the ephemeris of Heise & Verbunt (1988) ensures that the error in the computed phases is $\delta\mbox{$\phi_{\rm orb}$ }\le2\times10^{-3}$, which is of no concern for the purpose of the present paper. A phenomenological description of the photometric data used here has been included in the analysis of quasi-simultaneous BeppoSAX observations by Matt et al. (2000).