  We present spectroscopic observations of the runaway reddened star HD 34078
acquired during the last three years at Observatoire de Haute Provence and
McDonald Observatory as well as other spectra obtained since 1990. The drift of
the line of sight through the foreground cloud due to the large transverse
velocity of HD 34078 allows us to probe the spatial distribution of CH, CH+, CN
and DIBs carriers at scales ranging from about 1 AU up to 150 AU. In
particular, time variations in the equivalent width of absorption lines are
examined. A few past and recent high resolution observations of CH and CH+
absorption are used to search for line profile variations and to convert
equivalent widths into column densities.
  The data set reveals a 20 % increase in CH column density over the past 10
years with no corresponding variation in the column density of CH+ or in the
strengths of the 5780 and 5797 AA DIBs. CN observations indicate that its
excitation temperature has significantly increased from < 3.1 K in 1993 to 3.6
+- 0.17 K in 1998 while the CN column shows only a modest rise of ~ 12 +- 6 %.
The data also strongly suggest the existence of weak correlated variations in
CH and CH+ columns over periods of 6 - 12 months (or ~ 10 AU).
  These results are discussed in relation to CH+ production mechanisms. A dense
newly intervening clump is considered in order to explain the long-term
increase in the column density of CH, but such a scenario does not account for
all observational constraints. Instead, the observations are best described by
CH+ production in a photodissociation region, like that suggested for the
Pleiades and IC 348.
