Setteducati & Weaver (1962) estimated the angular diameter of Be 18
at 20 arcmin. The cluster is located relatively close to the
plane of the Milky Way and therefore
its field is strongly contaminated by the background/foreground stars.
To study the radial extension and richness of Be 18 we use data obtained with
the 0.9-m telescope. In Fig. 4 (click here) we show the observed luminosity function
for stars with the available V-band photometry. This figure shows
that our photometry becomes significantly incomplete for
V>21.0. On the other hand, the upper main-sequence of the cluster
terminates at
(see Figs. 1-3). Consequently, we limited our
attention to stars with 18.0<V<21.0 while studying the radial
extend
of Be 18. The cluster center was determined at 
,
where rectangular coordinates are given in pixels (as we describe in
Appendix A all photometric data used in this paper are available in
electronic form from editors of A&A). The average stellar density was
calculated in successive 47.6 arcsec (70 pixels) wide annuli around the
cluster center. Figure 5 (click here) shows the stellar surface density as a
function of distance from the cluster center. The density profile flattens
at radius 
arcsec. Adopting r=809 arcsec for the
cluster radius we derived 
stars/arcsec
for the average surface density
of the field stars. Adopting the above listed parameters we may estimate
that about 4060 stars with 18.0<V<21.0 populate
an upper main-sequence of the cluster. This estimate is in fact
just a conservative lower limit. First of all we made no correction
for the incompleteness of the photometry. Moreover, open clusters
often possess extended coronas harboring a significant fraction of
their member stars. Considering the number of upper-main sequence stars
possessed by Be 18 we may conclude that it is one of the richest objects,
if not the richest, in the whole sample of know old open clusters.