1 Introduction

Circumstellar CO radio line emission has proven to be a good measure of the gas mass-loss rate of evolved stars on the Asymptotic Giant Branch (AGB). Extensive surveys have been performed, and presently more than 500 objects have been detected (e.g., Knapp & Morris 1985; Zuckerman & Dyck 1986; Zuckerman et al. 1986; Leahy et al. 1987; Nguyen-Q-Rieu et al. 1986; Knapp et al. 1989; Loup et al. 1990; Margulis et al. 1990; Likkel et al. 1991; Bujarrabal et al. 1992; Nyman et al. 1992; Kastner et al. 1993; Olofsson et al. 1993; van der Veen et al. 1993; Volk et al. 1993; Bieging & Latter 1994; Hu et al. 1994; Kahane & Jura 1994; Sahai & Liechti 1995; Young 1995; Groenewegen et al. 1996; Kerschbaum et al. 1996c; Bieging et al. 1998; Groenewegen & de Jong 1998; Josselin et al. 1998; Knapp et al. 1998; Kerschbaum & Olofsson 1998). A significant fraction of this work was catalogued by Loup et al. (1993, hereafter L93). The surveys are dominated by high mass-loss rate, evolved objects. The low end of the mass-loss rate distribution, populated by objects located on the early AGB or at the beginning of the thermally pulsing AGB (i.e., the phase of episodic double-shell burning), has not been studied in that detail. We therefore started a major survey of circumstellar CO emission towards cool, O-rich (${\rm O/C}\gt 1$) semiregular (SRVs) and irregular (IRVs) variables.

The SRVs (types SRa and SRb) and the IRVs (Lb) are quite numerous groups of objects among the stars on the AGB. They can provide important constraints for theoretical mass-loss models due to their different pulsational behaviour compared to the more frequently studied Mira variables. In a series of papers (Kerschbaum & Hron 1992, 1994, 1995, hereafter SR_I, SR_IIa, and SR_III, respectively; Kerschbaum 1995, hereafter SR_IIb) the SRVs have been studied in a broad and systematic way. This made it possible to divide this inhomogeneous group of objects into physically distinct classes, based on stellar properties and galactic distribution. For the O-rich stars the subclasses "blue''-, "red''-, and "Mira''-SRVs were introduced. The "blue'' subclass contains objects with short periods (typically below 100$^{\rm d}$) and no apparent signs of mass-loss. The border between "red''- and "Mira''-SRVs is mainly defined by the period ($P<200^{\rm d}$ for "red'', and $P\gt 200^{\rm d}$ for "Mira''). The "blue''-SRVs seem to be on the early AGB, while the other two groups are on the thermally pulsing AGB. Jura & Kleinmann (1992) reached the same conclusions concerning the galactic distribution of these stars.

Work on the IRVs is complicated by the larger contamination by poorly observed late-type variables (to some extent this applies also to the SRVs, Lebzelter et al. 1995). Kerschbaum et al. (1996b, hereafter Lb_I) found that their sample of visually bright IRVs displays infrared properties very similar to the SRVs. There may be a slightly larger "contamination'' with non-AGB giants than in the case of the SRVs, but the AGB objects seem to resemble the "blue'' and the "red'' SRVs. In a paper by Kerschbaum et al. (1996a) based on NIR- and IRAS-photometry, full spectral energy distributions, mass-loss rates, and galactic distributions were analyzed, and the similarity between the IRVs and SRVs was further demonstrated.

A more detailed study of the circumstellar properties of O-rich SRVs started with the work of Kahane & Jura (1994, hereafter K94) who published high quality CO(J=1-0 and 2-1) line observations of eleven nearby objects. They came to the conclusion that these objects have mass-loss properties quite similar to those of longer-period O-rich Miras. Kerschbaum et al. (1996c, hereafter SR_IV) extended this work considerably in their study of O-rich SRVs. The majority of their detected objects, covering both small and longer periods, are low mass-loss rate objects (), and low expansion velocity envelopes (the mean value is $\sim$8 kms^-1). A significant number of C-rich SRVs and IRVs was included in the survey of Olofsson et al. (1993).

O-rich IRVs were for the first time studied in circumstellar CO by Kerschbaum & Olofsson (1998). They found that their mass-loss properties are very similar to those of O-rich SRVs and (optically bright) Miras, i.e., at least in this mass-loss rate range the stellar mass-loss properties are not strongly influenced by the pulsational behaviour of the star.

In this paper we present a catalogue and spectra of all CO radio line data that we have obtained towards O-rich SRVs and IRVs. In total 66 sources have been detected, 43 SRVs and 23 IRVs. Only a minor fraction of the data have already been presented in Kerschbaum et al. (1996c) and Kerschbaum & Olofsson (1998). We also discuss some of the quantitative results that can be derived directly from the data.