We present phase-resolved spectrophotometric observations of VV Puppis obtained during two different states of accretion. We confirm the detection of cyclotron lines from emission regions at both poles (Wickramasinghe et al. 1989) having significantly different field strength of B1 ≃ 31 MG and B2 ≃ 54 MG. Our phase-resolved data allowed the detection of phase-dependent wavelength shifts of the cyclotron lines from the main accretion pole which is due to the varying aspect of the observer. A corresponding motion of the cyclotron lines from the secondary pole appears likely. Compared to 1984, the cyclotron lines from the main pole appeared redshifted in 1989, during an episode of higher system brightness. This shift can be explained assuming either different locations of the accretion spot and, hence, variations of the magnetic field, or variations of the plasma temperature both triggered by variations of the mass accretion rate. The cyclotron lines from the second pole do not show such changes suggesting that the coupling region for the weakly accreting pole is more or less stationary whereas the coupling region of the main pole varies in space, most likely depending on the mass accretion rate.Key words: cataclysmic variables -AM Herculis binaries -stars: individual (VV Pup) -cyclotron radiation -stars: magnetic field AAA subject classification: 119; 120; 122
IntroductionVV Puppis was recognized as third AM Herculis binary by the detection of strong and periodic variable linear and circular polarization by Tapia (1977). A high orbital inclination of i ∼ 75• and a high "southern" colatitude of the main accretion region of δ ∼ 150• lead to a long selfeclipse of this main spot by the white dwarf itself as the system rotates. Visvanathan and Wickramasinghe (1979) and Stockman et al. (1979, hereafter SLB) detected intense low-frequency modulations of the continuum spectra during the bright phase, with the minima originally explained as cyclotron absorption troughs in a field of ∼ 30 MG. Later, when detailed cyclotron models became available, the maxima in intensity were interpreted as cyclotron line emission and the field strength was redetermined to 31.5 -32.0 MG (Wickramasinghe & Meggitt 1982;Barrett & Chanmugam 1985).Photometric variations and nonvanishing circular polarization during the faint phase, when the main accreting pole is out of view, have been interpreted by Liebert and Stockman (1979) as due to accretion in the vicinity of the second pole. Wickramasinghe et al. (1989, hereafter WFB) confirmed this supposition by the detection of two independent systems of cyclotron lines originating from both accretion regions. The field strength in the secondary region was found to be surprisingly high, B 2 ≃ 56 MG, and the newly determined field strength for the primary pole, B 1 = 30.5 MG, was lower than during the initial observations. These findings have been interpreted assuming that accretion occurs at the footpoint of the same closed field line in a decentered dipole field (d off ≃ 0.1R wd ). The lower value o...