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Abstract. We report the detection of an Hα emission-line structure in the upstream side of the planetary nebula NGC 40, which is predicted by numerical simulations, and which is attributed to Rayleigh-Taylor instability. Such a Rayleigh-Taylor instability is expected to occur at early stages of the interaction process between the interstellar medium (ISM) and a fast moving planetary nebula, as is the case for NGC 40. We resolved the Rayleigh-Taylor instability "tongues", as well as the flatness of the nebula around the "tongues", which results from the deceleration by the ISM.

Using HST STIS, we have detected far-ultraviolet nuclear activity in the giant elliptical galaxy NGC 1399, the central and brightest galaxy in the Fornax I cluster. The source reached a maximum observed far-UV luminosity of $1:2 ; 10 39 ergs s À1 in 1999 January. It was detectable in earlier HST archival images in 1996 (B band ) but not in 1991 (V band ) or 1993 (UV ). It faded by a factor of $4 by mid-2000. The source is almost certainly associated with the low-luminosity AGN responsible for the radio emission in NGC 1399. The properties of the outburst are remarkably similar to the UV-bright nuclear transient discovered earlier in NGC 4552 by Renzini and coworkers. The source is much fainter than expected from its Bondi accretion rate (estimated from Chandra high-resolution X-ray images), even in the context of ''radiatively inefficient accretion flow'' models, and its variability also appears inconsistent with such models. High spatial resolution UV monitoring is a valuable means to study activity in nearby LLAGNs.

iivans=astro.as.utexas.edu ; jodie=astro.as.utexas.edu ; cyndi=astro.as.utexas.edu