  Our aim is to investigate the possible physical association between
consecutive coronal mass ejections (CMEs). Through a statistical study of the
main characteristics of 27761 CMEs observed by SOHO/LASCO during the past 20
years. We found the waiting time (WT) or time elapsed between two consecutive
CMEs is $< 5$ hrs for 59\% and $< 25$ hrs for 97\% of the events, and the CME
WTs follow a Pareto Type IV statistical distribution. The difference of the
position-angle of a considerable population of consecutive CME pairs is less
than $30^\circ$, indicating the possibility that their source locations are in
the same region. The difference between the speed of trailing and leading
consecutive CMEs follows a generalized Student t-distribution. The fact that
the WT and the speed difference have heavy-tailed distributions along with a
detrended fluctuation analysis shows that the CME process has a long-range
dependence. As a consequence of the long-range dependence, we found a small but
significative difference between the speed of consecutive CMEs, with the speed
of the trailing CME being higher than the speed of the leading CME. The
difference is largest for WTs < 2 hrs and tends to be zero for WTs > 10 hrs,
and it is more evident during the ascending and descending phases of the solar
cycle. We suggest that this difference may be caused by a drag force acting
over CMEs closely related in space and time.
