COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      NUMBER 819

	                                Konkoly Observatory
        	                        Budapest
                	                1973 August 15


    ON THE RELATIONSHIP BETWEEN THE I-K COLOUR INDEX AND
     THE INTRINSIC POLARIZATION OF THE LATE-TYPE STARS



    As is well known (1,2), the intrinsic polarization of the M-
type stars changes along with the brightness variation of the stars
and also depends on their spectral type. The observations also show
that within the same spectral class the intrinsic polarization of the
stars can be different.
    We made a detailed investigation of the observational data and
found an interesting relation between the I-K colour index and the 
intrinsic polarization of the M-type stars. Serkowski's polarimetric 
data (3) and the I-K colour indices of the CIT-catalogue (4) formed the
basis of the present investigation. The effective wavelength of the I
magnitude is at 0.9 microm and that of the K magnitude is at 2 microm. 
In each spectral class (from M0 to M8) we selected the stars whose 
intrinsic polarizations at maximum light exceeded the average value of 
the polarization of the corresponding spectral class (p > avr.p).

                      [FIGURE 1]
               
    In Fig. 1 we plotted the I-K average colour indices of the highly 
polarized stars (dots) and that of other stars (crosses) against
the spectral classes.
    As can be concluded from Figure 1, in one and the same spectral
class later than M1 the average colour indices I-K of the stars having
intrinsic polarizations are systematically larger than that of the
stars with non-polarized light. We note that the I-K colour index of
a star is generally between the two lines of Fig. 1 if its intrinsic
polarization is smaller than the average polarization p of the 
spectral class which the star belongs to.
    These facts indicate that the stars with intrinsic polarizations
show I-K excess compared with the other stars of the same spectral
class.

    Using the data of (3,4) we obtained the following relation 
between I-K and log p.

               I-K = 4.65 + 2.58 log p
In connection with this a further qualitative analysis can be made.
In the CIT-catalogue the stars showing brightness variations in I-
and/or K-spectral range are marked. Further on we call these stars I-,
K- and IK-type variables, respectively.
    The frequency of the I-, K- and IK-type variables, as a function
of I-K was investigated. The frequency distributions are shown in 
Figure 2. As can be seen from the Figure, the maxima of the frequency 
distributions of the K- and IK-type variables are shifted towards the
higher I-K values. Compared with the formula obtained this means that
the occurrence of intrinsic polarization is more probable among the
K-type variables. The calculations reveal that the frequency of the
stars having intrinsic polarization is, indeed, 1.5 times greater 
among the K- and IK-type than among the I-type variables. This relation
is also in accordance with the fact that near the galactic equator
(|b| < 25deg) the intrinsic polarization of the stars and the ratio n_K/n_I
of the numbers of the K- and I-type variables are 1.5-2.0 times greater
than the corresponding quantities for the stars at high galactic
latitudes (|b| > 25deg).
    Since the intrinsic polarization of a late-type variable increases
with the decrease of the brightness of the star, it would be worth 
investigating the relation between the polarization and the I-K colour
index during the light variation separately for each late-type variable.

                        [FIGURE 2]


      L. SZABADOS          R.A. VARDANIAN
    Konkoly Observatory  Byurakan Observatory



    References:
1   T. Gehrels, A. Kruszewski, K. Serkowski, A.J., 73, 677, 1969. [BIBCODE 1968AJ.....73..677K ]
2   R.A. Vardanian, Astrofizika, 6, v. 1, 77, 1970. [BIBCODE 1970Afz.....6...77V ]
3   K. Serkowski, preprint, Lun.Plan.Lab. Arizona, 1971.
4   G. Neugebauer, R.B. Leighton, Two-Micron Sky Survey, Cal.Tech.
    Pasadena, 1968.