Orbital period changes in V2051~Oph
Session 55 -- Interacting Binaries: CVs and XRBs
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [55.08] Orbital period changes in V2051~Oph

L.Triplett, R.Baptista (STScI), J.E.Steiner (IAG/USP), F.J.Jablonski (INPE and U.Texas)

We report the identification of orbital period changes in the short-period eclipsing cata\-clysmic variable V2051~Oph. We used times of mid-eclipse collected from the literature and timings from our unpublished data to construct an observed-minus-calculated eclipse time diagram covering 13 years of observations (1979-92).

At present there is no evidence of secular orbital period (P$_{\rm orb}$) decrease. The uncertainty in the quadratic fit sets an upper limit for period changes of P$_{\rm orb}$/\.{P}$\:> 5.9 \times 10^7$ yr. Assuming a secondary mass of M$_2 = 0.2\:{\rm M}_\odot$ this yields \.{M}$_2 < 3.4 \times 10^{-9} \:{\rm M_{\odot} \,yr}^{-1}$ as an upper limit for the mass transfer rate in the system.

The data present cyclical variations that can be fitted by a linear plus sinusoidal function with periods 4.7 yr and 7.6 yr. The statistical significance of these periods by an F-test are, respectively, 93\% and 97\%. The multiplicity of possible solutions suggest that V2051~Oph exhibits cyclical but non-periodic P$_{\rm orb}$ changes, which could be explained in terms of a magnetic activity cycle in its secondary star. An incremental radius variation of $\Delta R_2/R_2 \simeq 10^{-6}$ is required to explain the observed amplitude of the cycle. This is of the same order of the values found for EX~Hya (Warner 1988) and V4140~Sgr (Baptista, Jablonski \& Steiner 1992), and is an order of magnitude smaller than the calculated values for systems above the period gap (implying proportionally smaller magnetic field strenghts).

This result, along with the evidence from V4140~Sgr and EX~Hya (and possibly also Z~Cha), suggests that if the magnetic cycle hypothesis is correct, the secondaries of cata\-clysmic variables below the period gap also have magnetic fields, although weaker than those of secondaries above the gap. This may serve to constraint current models attempting to explain the existence of the period gap.