A star like the Sun becomes a planetary nebula towards the end of its life, when the envelope ejected during the earlier giant phase becomes photoionized as the surface of the remnant star reaches a temperature of approximately 30,000 K. The spherical symmetry of the giant phase is lost in the transition to a planetary nebula, when non-spherical shells and powerful jets develop. Molecules that were present in the giant envelope are progressively destroyed by the radiation. The water-vapour masers that are typical of the giant envelopes therefore are not expected to persist in planetary nebulae. Here we report the detection of water-maser emission from the planetary nebula K3-35. The masers are in a magnetized torus with a radius of about 85 astronomical units and are also found at the surprisingly large distance of about 5,000 astronomical units from the star, in the tips of bipolar lobes of gas. The precessing jets from K3-35 are probably involved in the excitation of the distant masers, although their existence is nevertheless puzzling. We infer that K3-35 is being observed at the very moment of its transformation from a giant star to a planetary nebula.

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars 1 . Most stars, however, form in multiple systems 2-5 , where the presence of a close companion could affect the structure of the disk 6-8 and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiter's orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars 9-12 . The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.L1551 is a molecular cloud in Taurus that is known to be undergoing intensive star formation, although restricted to lowmass stars (that is, with masses comparable to or smaller than that of the Sun). The embedded infrared source L1551 IRS5 (ref. 13) is believed to be associated with a very young stellar source. Its bolometric luminosity is ϳ30 solar luminosities 14 and it is embedded in a dense envelope of molecular gas and dust that extends over ϳ10 3 -10 4 AU (refs 15, 16). The continuum millimetre emission, when observed with angular resolutions of ϳ1 arcsec, has been interpreted as originating from heated dust in a protoplanetary disk with dimensions of about 100 AU (refs 17, 18).In the centimetre wavelengths, there is compact radio continuum emission with elongated morphology 19,20 that aligns with the largescale bipolar molecular outflow 21 . This centimetre radiation is known to be free-free emission that originates in the ionized outflowing gas 22 . However, when observed with subarc second angular resolution, the core of the centimetre emission from L1551 IRS5 breaks into two compact components separated by 0.3 arcsec, which have been interpreted as being either a protobinary system 19 or the inner ionized edges of a gas and dust toroid around a single star 20 . As the millimetre continuum emission, tracing the dust, has been interpreted as coming from a single disk, the singlestar interpretation has been favoured recently. However, there are several observations that suggest that two independent outflow systems emanate from L1551 IRS5 (refs 23-25), favouring a binary nature for the source. Furthermore, observations at 2.7-mm (ref. 26) with angular resolution of about 0.5 arcsec have been used to argue for the presence of two unresolved disks in L1551 IRS5.Our observations were made with the Very Large Array of the National Radio Astronomy Observatory in its highest angular resolution A configuration. The 7-mm observations were made in 1997 January 10, and the 3.6-cm observations were made in 1996 December 10. As the observations are separated b...

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