1. Introduction

Maser emission from the rotational transitions of the vibrationally excited states of ²⁸SiO (SiO hereafter) has been detected in more than 500 evolved stars and also in a few star-forming regions. The variability of the SiO masers was established in the very first works on this type of emission (see e.g. Hjalmarson & Olofsson 1979; Clark et al. 1982). Soon, this variability was studied by several projects monitoring these masers from a sample of sources. Up to date, the most complete and systematic of these monitorings are those by Lane (1982), Nyman (1985), and Martínez et al. (1988). They established some characteristics of the SiO maser variability, such as the periodic variations in Mira type stars, the systematic occurrence of the SiO maxima at phase $\sim0.1-0.2$ with respect to the optical maxima, and the poor reproducibility of the SiO light curves. However, more systematic work covering longer periods of time with the necessary tightness, and including more sources, is necessary to shed light on other questions about this variability; in particular, on the correlation between the SiO and near infrared (NIR) light curves, the variation pattern of the SiO maser profiles, and the eventual presence of longer secondary periods.

In July 1984, a long-term short-spaced monitoring of well known SiO maser sources was started at the 13.7 meter radiotelescope of the Centro Astronómico de Yebes (CAY) of the Spanish National Astronomical Observatory (OAN). Two SiO maser lines have been observed, the J=1-0 rotational transitions of the v=1 and v=2 vibrationally excited states, whose respective rest frequencies are 43122.024 and 42820.432 MHz. At the beginning of the project, 14 sources were observed every about three weeks, but only in the v=1 line. One year later, we started monitoring also 10 of these sources in the v=2 line. By mid 1987 two additional sources were included, all 16 sources being observed in the two transitions since then. Finally, in January 1989 five more sources were added to the list, for a total of 21 sources. The selected sources cover all types of SiO maser emitters known to date. In Table 1, we list the objects included in our monitoring and some of their characteristics, namely the type of source, the spectral type, and the period and amplitude of the optical light curve ( $\Delta m_v$), taken from the 4$^{\rm th}$ edition of the General Catalogue of Variable Stars (Kholopov et al. 1985). The starting dates for the v=1 and v=2 observations, number of spectra presented in this paper, mean time-interval of the sampling, and other characteristics of the monitoring performed for each object are presented in Table 2.

   

Table 1: Observed objects: characteristics

	name	spectral type	Period	$\Delta m_v$	comments
			(days)
Supergiants
	VY CMa	M5		3.1
	$\mu$ Cep	M2	730?	1.7
	VX Sgr	M4-M10	732	7.5
Semiregulars
	GY Aql	M6-M8	204?(1)	$\>$6
	RT Vir	M8	155?	1.3
Mira Variables
	R Aqr	M5-M9	387	6.6	Symbiotic
	R Aql	M5-M9	284	6.5
	TX Cam	M8-M10	557	6.1
	R Cnc	M6-M9	361	5.7
	R Cas	M6-M10	430	8.8
	o Cet	M5-M9	332	6.8	Mira
	$\chi$ Cyg	S6-S10	408	10.9	S-type
	U Her	M7-M10	406	6.9
	W Hya	M7-M9	361	3.9	SRa ?
	X Hya	M7-M9	301	6.4
	R Leo	M6-M10	310	6.9
	R LMi	M7-M10	372	6.9
	IK Tau	M6-M10	470	5.7	NML Tau
OH/IR Stars
	IRC +10011	M8	660		WX Psc
	OH 26.5+0.6
Young objects
	Orion IRc2
Notes: (1) New optical data suggest a period of $\sim$ 400 days.

   

Table 2: Observational parameters

name	Observational	Velocity	PA $^{\rm (1)}$	EA $^{\rm (2)}$	Sampl. $^{\rm (3)}$	Num. of	Starting date
	coordinates	range	($^{\circ}$)	($^{\circ}$)	(days)	spectra	yymmdd	Fig.#
	$\alpha$ (1950)     $\delta$ (1950)	(km s-1)	v=1;2	v=1;2	v=1;2	v=1;2	v=1;2
VY CMa	07 20 55.0 -25 40 12	-29:61 $^{\rm (4)}$	-7;-8	23;23	28;36	70;51	840628;850709	2
$\mu$ Cep	21 41 58.5 +58 33 01	3:47	102;91	56;50	28;34	33;28	870522;870614	3
VX Sgr	18 05 03.0 -22 13 55	-37:53 $^{\rm (5)}$	9;-2	26;26	27;31	73;30	840628;870523	4
GY Aql	19 47 25.0 -07 44 30	11:55	15;3	39;38	27;38	73;52	840713;850709	5
RT Vir	13 00 05.8 +05 27 14	-5:39	42;8	33;52	27;35	60;28	850528;870522	6
R Aqr	23 41 14.0 -15 33 42	-48:4	-5;-13	33;30	28;36	72;51	840627;850710	7
R Aql	19 03 57.7 +08 09 08	24:68	-34;-36	47;45	39;46	14;12	890110;890111	8
TX Cam	04 56 43.0 +56 06 48	-11:33	100;101	57;57	27;46	74;29	840613;870522	9
R Cnc	08 13 48.6 +11 52 52	-5:39	11;10	60;60	36;50	14;12	890110;890111	10
R Cas	23 55 52.0 +51 06 37	4:48	90;56	61;30	26;38	74;29	840613;870522	11
o Cet	02 16 49.0 -03 12 13	23:67	29;9	37;44	24;37	77;50	840626;850709	12
$\chi$ Cyg	19 48 38.4 +32 47 10	-14:30	64;64	58;58	36;53	14;12	890109;890110	13
U Her	16 23 35.0 +19 00 18	-36:8	-40;-19	57;64	27;33	72;51	840726;850709	14
W Hya	13 46 12.0 -28 07 06	17:61	-7;7	20;21	27;34	72;50	840726;850709	15
X Hya	09 33 06.7 -14 28 05	5:49	19;16	31;32	50;48	14;11	890110;890204	16
R Leo	09 44 52.2 +11 39 40	-23:21	1;-11	60;58	28;36	72;50	840726;850709	17
R LMi	09 42 35.0 +34 44 34	-22:22	65;60	64;67	27;38	73;48	840726;850709	18
IK Tau	03 50 43.6 +11 15 32	13:57	-35;-47	49;35	25;35	68;50	850124;850709	19
IRC +10011	01 03 48.0 +12 19 51	-14:30	50;45	26;32	30;48	32;25	870522;870703	20
OH 26.5+0.6	18 34 52.5 -05 26 37	5:49	17;17	41;40	46;56	12;10	890119;890110	21
Orion IRc2	05 32 47.0 -05 24 23	-39:51	6;-7	42;42	24;37	77;58	840628;840810	22
Notes: $^{\rm (1)}$	Mean parallactic angle of the observations.
$^{\rm (2)}$	Mean elevation angle of the observations.
$^{\rm (3)}$	Mean sampling interval without taking into account the 180 days break in 1988.
$^{\rm (4)}$	$V_{\rm lsr}$ range from +6 to +38 km$\,$s$^{\rm -1}$ before December 1987.
$^{\rm (5)}$	$V_{\rm lsr}$ range from -14 to +30 km$\,$s$^{\rm -1}$ before December 1987.

The goals of this work have been to obtain a relatively short-spaced sampling in time ($\sim$ 3 weeks) of the two selected transitions during a period of time covering several stellar cycles for Mira-type variables (5 to 7 in most cases). We succeeded on those requirements during the entire period except for a few breaks of less than 100 days, and a break of 180 days in 1988 due to major works on the 45 GHz receiver of the CAY-13.7 m radiotelescope. Thus, we are able to present a data set rather unique both by its regular time sampling and the long period of time covered. Here we present more than 1800 spectra, to be compared with the less than 300 presented in the monitorings by Lane (1982) and Nyman (1985). Preliminary results of this project, corresponding to the first two and a half years of v=1 observations, and one and a half years of v=2 observations, were published by Martínez et al. (1988).

This paper constitutes the general presentation of our SiO data set, that is also compared with the optical and NIR variability curves of the stars when available. In Sect. 2 we describe the main characteristics of the observing system. Section 3 is devoted to a detailed discussion of the observational procedure adopted in order to get a relative calibration of the data as good as possible over the entire project. The general description of the data and how they are presented in this paper can be found in Sect. 4, were the major results are also outlined. Section 5 is devoted to brief descriptions of the results obtained for the individual sources. Finally, the conclusions are summarized in Sect. 6.