Spectral classification of the cool giants in symbiotic systems

We derive the spectral types of the cool giants in about 100 symbiotic syst ems. Our classification is mainly based on near IR spectra in order to avoi d the contamination of the spectrum by the nebula and the hot component in the visual region. The accuracy of our spectral types is approximately one spectral subclass, similar to previous near IR classification work, and muc h better than visual spectral type estimates. Strong, intrinsic spectral type variations (> 2 spectral subtypes) are only seen in systems containing pulsating mira variables. We present a catalogue of spectral types for cool giants in symbiotic syste ms which also includes determinations takes from the literature. The catalo gue gives spectral types for the cool giants in about 170 systems which is nearly the full set of confirmed symbiotics. Based on our classifications we discuss the distribution of spectral types of the cool giants in galactic symbiotic binaries. We find that the spectra l types cluster strongly between M3 and M6, with a peak at M5. The distribu tion of systems with a mira variable component peaks even later, at spectra l types M6 and M7. This is a strong bias towards late spectral types when c ompared to red giants in the solar neighbourhood. Also the frequency of mir a variables is much larger among symbiotic giants. This predominance of ver y late NI-giants is symbiotic systems seems to indicate that large mass los s is a key ingredient for triggering symbiotic activity on a white dwarf co mpanion. Further we find for symbiotic systems a strong correlation between the spec tral type of the cool giant and the orbital period. In particular we find a tight relation for the minimum orbital period for symbiotic systems with r ed giants of a given spectral type. This limiting line in the spectral type - orbital period diagram seems to be equivalent with the relation R less t han or equal to l(1)/2, where R is the radius of the red giant and l(1) the distance from the center of the giant to the inner Lagrangian point L-1. T his correlation possibly discloses that symbiotic stars are - with probably only one exception in our sample - well detached binary systems.