The pulsating planetary nebula central star RXJ2117+3412 (V=13.2mag) is the best example of the transition phase between planetary nebulae and white dwarfs. It oscillates with many g-modes excited simultaneously, with periods of 500-1200s and amplitudes of 1-5mma. RXJ2117 has been observed during three successive multisite photometric campaigns, including XCOV8 in September 1992 and XCOV11 in August 1994. The asteroseismological analysis, based on 37 identified l=1 modes, has yielded a number of fundamental parameters of the star. The average rotation period is 1.16day. The rotational splitting decreases with the increasing g-mode period, a clear indication of a non-solid body rotation inside RXJ2117. The average period spacing is 21.618s, which yields a total mass of 0.56 (+0.02,-0.04) Msun. From the mode trapping cycle, one infers that the He-rich envelope must be at least 0.013 Mstar thick, and possibly as thick as 0.078 Mstar. The luminosity derived from asteroseismology is log(L/Lsun)= 4.05 (+0.23,-0.32) and the distance is 760+-235pc. The results of this asteroseismological study of RXJ2117 are published in Vauclair et al. (2002).

The main purpose of the new WET observations proposed here is to measure the rate of period change of RXJ2117. This will provide a direct determination of the evolutionary timescale of the star. We want to measure dP/dt for as many modes as possible. As both trapped and non-trapped modes are seen (and identified in previous work), we should be able to determine both the timescale for contraction of the outer layers of the star (from the trapped modes) and the timescale for cooling of its core (from the non-trapped modes). If we succeed in measuring dP/dt for two modes of the same triplet, we will also be able to constrain the rate of secular variation of rotation period.

Until now, the rate of period change has been determined only for one DOV/PNNV pulsator, namely for PG1159-035. In their study Costa et al. (1999) have not used an O-C diagram, but instead they have determined dP/dt by directly measuring pulsation period at several different epochs. With 4 epochs and the timebase of 10 years they have been able to measure dP/dt with accuracy of 25sigma (maximum likelihood method), later improved to 400sigma (so-called modified O-C method). The rate of period change obtained by Costa et al., dP/dt=1.3x10^{-10}s/s, is an order of magnitude faster than predicted by a standard evolutionary theory. This curious result makes determination of dP/dt for another similar star an important and urgent project. We need to know if PG1159 is a typical member of its class or not.

We intend to apply Costa's et al. direct dP/dt measurement method to RXJ2117. This star is evolutionary younger and therefore is expected to evolve as fast as PG1159 or faster. Multisite photometry of RXJ2117 obtained in 1992, 1993 and 1994 is already published. A WET run in 2004 will give us a total timebase of 12 years. Judging from PG1159 experience, this should be enough to determine dP/dt for RXJ2117.

The secondary goal of the WET run is to detect new modes, not seen during previous campaigns. As in many other white dwarfs (e.g. GD358, G29-38), not all the modes of RXJ2117 are visible in any given moment and several observing seasons are needed to establish the complete set of pulsation frequencies. The new frequencies will help to refine the determination of the trapping cycle. This would put stronger constraints on the future detailed modeling of the star. New modes will also supplement uncomplete l=1 triplets, allowing more rotational splittings to be measured. This will give the possibility of detecting the signature of the mode trapping on the rotational splitting, which will lead to a better understanding of internal rotation of RXJ2117.

RXJ2117 is a relatively bright object (V=13.2mag) and can be observed even by the smallest telescopes in the WET network. The planetary nebulae surrounding the star has very low surface brightness. Because of that, excellent quality data can be obtained even with the PMT photometers.

Vauclair, G., Moskalik, P., et al. 2002, A&A, 381, 122.