The pulsating nucleus of the planetary nebula Longmore 4

CCD photometry shows that the central star of the planetary nebula Longmore 4 is a mulfiperiodic, nonradially pulsating variable. Its light curves show an amplitude that can reach 0.1 mag (peak to peak) and a strong periodicity near 31 min, but with considerable interference among numerous pulsation modes. Power spectra reveal nearly a dozen significant individual periods, lying in groups around 875, 1560, and 1850 s. The optical spectra of Lo 4 and K 1-16, the other known pulsating planetary nucleus, and of the four known pulsating GW Vir white dwarfs, are all dominated by features of He II, C IV, and O VI, indicating extremely hot (T_eff ≳ 10⁵ K), hydrogen-deficient, and Carbon- and oxygen-rich surface layers. The apparent uniformity of the chemical compositions supports a pulsation mechanism proposed by Starrfield et al. involving cyclical ionization of carbon and/or oxygen. The longer oscillation periods of Lo 4 and K 1-16, along with the presence of planetary nebulae, is consistent with an evolutionary sequence in which they are the progenitors of pulsating GW Vir variables. In fact, continued photometric observations of Lo 4 should be capable of revealing the predicted decreases in the pulsation periods due to the star's rapid evolutionary contraction.