Astron. Astrophys. 350, L27–L30 (1999) ASTRONOMY AND ASTROPHYSICS Letter to the Editor ISO  observations of dust formation in Sakurai’s object Monitoring the mass loss of a very late Helium flash star F. Kerber 1,2 , J.A.D.L. Blommaert 3 , M.A.T. Groenewegen 4 , S. Kimeswenger 2 , H.U. K¨ aufl 5 , and M. Asplund 6 1 Space Telescope European Coordinating Facility, European Southern Observatory, 85748 Garching, Germany 2 Institut f ¨ ur Astronomie der Universit¨ at Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria 3 ISO Data Centre, ESA, Villafranca, 28080 Madrid, Spain 4 Max-Planck-Institut f ¨ ur Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany 5 European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany 6 Uppsala Astronomical Observatory, Box 515, 75120 Uppsala, Sweden Received 27 May 1999 / Accepted 2 September 1999 Abstract. We present ISOCAM observations of Sakurai’s ob- ject (V4334 Sgr) covering the wavelength range of 4 to 15 μm in seven filters. The photometry shows that in the period from February 1997 to February 1998 the flux over the whole wave- length range has increased by a factor of about ten. Combined with ground-based data we conclude that this increase is the result of mass loss from Sakurai’s object and the formation of hot dust around it. Using a spherically symmetric dust radia- tive transfer model we obtain a quantitative result of a variable and increasing mass loss rate reaching some 10 -7 M  /yr, a value not uncommon among stars during the Asymptotic Giant Branch (AGB) evolution. This is in agreement with the notion that Sakurai’s object is retracing its own evolutionary history as a consequence of a very late Helium flash. We conclusively demonstrate that significant mass loss is associated with such an event and foster the link to the other few known examples of final Helium flashes. Key words: stars: AGB and post-AGB – stars: evolution – stars: circumstellar matter – ISM: planetary nebulae: individual: Saku- rai’s Object (V4334 Sgr) 1. Introduction Sakurai’s object (V4334 Sgr) most likely is a star undergoing a final Helium flash, the first example since V605 Aql/A 58 in 1919. Details of its discovery and evolution are found in Duer- beck et al. (1996, 1997), Kerber et al. (1999a) and references therein. This is the first such event that can be studied with Send offprint requests to: F. Kerber (fkerber@eso.org)  ISO is an ESA project with instruments funded by ESA Mem- ber States (especially the PI countries: France, Germany, The Nether- lands and the United Kingdom) and with the participation of ISAS and NASA. modern technology and at wavelengths other than optical. Most recently two studies of the old planetary nebula (PN) surround- ing Sakurai’s object (Kerber et al. 1999a; Pollacco 1999) have demonstrated that a very late Helium flash is under way, i.e. one that happened when the star was highly evolved (T ≈ 100 000 K) and had already entered the white dwarf cooling track. This re- sult is in full agreement with the quantitative analysis of Asplund et al. (1999) who show that Sakurai’s object is hydrogen-poor and hydrogen is still being removed from the photosphere as it is ingested and burned, whereas s-process elements become more abundant. The theory of a very late Helium flash (Iben et al. 1983) pre- dicts that such stars will return to the AGB and retrace their own post-AGB evolution for a second time (“born-again”). This will lead to the formation of a second PN composed of hydrogen- poor material and dust (Harrington 1996). The prototypes of these PNe are A 30 and A 78 (Jacoby & Ford 1983; Hazard et al. 1980) but also V605 Aql/A 58 (Seitter 1987), GIJC-1 (Gillett et al. 1989) in M 22 and IRAS 15154-5258 (Manchado et al. 1989) belong to this class. Therefore both theory and the known examples of very late Helium flashes suggest that significant mass loss is associated with such an event. 2. Observations and data reduction Sakurai’s object has shown fast and massive changes in its spec- trum as carbon bearing molecules have formed (Asplund et al. 1997; Kerber et al. 1997) very much comparable to V605 Aql (Clayton & de Marco 1997). We were the first to discover a strong infrared excess (Kimeswenger et al. 1997) as a direct result of mass loss (Kerber et al. 1999b). Using general and dis- cretionary time on ISO we have obtained a homogeneous set of photometric observations spanning the full ISOCAM (Cesarsky et al. 1996) LW wavelength range and covering a full year from February 1997 to 1998 in four epochs, closely monitoring the LETTER