Context. Phosphorus-bearing compounds have only been studied in the circumstellar environments of the asymptotic giant branch star IRC 10 216 and the protoplanetary nebula CRL 2688, both carbon-rich objects, and the oxygen-rich red supergiant VY CMa. The current chemical models cannot reproduce the high abundances of PO and PN derived from observations of VY CMa. No observations have been reported of phosphorus in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars. Aims: We aim to set observational constraints on the phosphorous chemistry in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars, by focussing on the Mira-type variable star IK Tau. Methods: Using the IRAM 30 m telescope and the Submillimeter Array, we observed four rotational transitions of PN (J = 2-1,3-2,6-5,7-6) and four of PO (J = 5/2-3/2,7/2-5/2,13/2-11/2,15/2-13/2). The IRAM 30 m observations were dedicated line observations, while the Submillimeter Array data come from an unbiased spectral survey in the frequency range 279-355 GHz. Results: We present the first detections of PN and PO in an oxygen-rich asymptotic giant branch star and estimate abundances X(PN/H2) ? 3 × 10-7 and X(PO/H2) in the range 0.5-6.0 × 10-7. This is several orders of magnitude higher than what is found for the carbon-rich asymptotic giant branch star IRC 10 216. The diameter (?0.''7) of the PN and PO emission distributions measured in the interferometric data corresponds to a maximum radial extent of about 40 stellar radii. The abundances and the spatial occurrence of the molecules are in very good agreement with the results reported for VY CMa. We did not detect PS or PH3 in the survey. Conclusions: We suggest that PN and PO are the main carriers of phosphorus in the gas phase, with abundances possibly up to several 10-7. The current chemical models cannot account for this, underlining the strong need for updated chemical models that include phosphorous compounds. This work is partially based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
