  Samples of pristine Solar System material found in meteorites and
interplanetary dust particles are highly enriched in (15)N. Conspicuous
nitrogen isotopic anomalies have also been measured in comets, and the
(14)N/(15)N abundance ratio of the Earth is itself larger than the recognised
pre-solar value by almost a factor of two. Ion--molecules, low-temperature
chemical reactions in the proto-solar nebula have been repeatedly indicated as
responsible for these (15)N-enhancements. We have searched for (15)N variants
of the N2H+ ion in L1544, a prototypical starless cloud core which is one of
the best candidate sources for detection owing to its low central core
temperature and high CO depletion. The goal is the evaluation of accurate and
reliable (14)N/(15)N ratio values for this species in the interstellar gas. A
deep integration of the (15)NNH+ (1-0) line at 90.4 GHz has been obtained with
the IRAM 30 m telescope. Non-LTE radiative transfer modelling has been
performed on the J=1-0 emissions of the parent and (15)N-containing dyazenilium
ions, using a Bonnor--Ebert sphere as a model for the source. A high-quality
fit of the N2H+ (1--0) hyperfine spectrum has allowed us to derive a revised
value of the N2H+ column density in L1544. Analysis of the observed N(15)NH+
and (15)NNH+ spectra yielded an abundance ratio N[N(15)NH+]/N[(15)NNH+] = 1.1
+/- 0.3. The obtained (14)N/(15)N ratio is ~ 1000 +/- 200, suggestive of a
sizeable (15)N depletion in this molecular ion. Such a result is not consistent
with the prediction of present nitrogen chemical models. As chemical models
predict large (15)N fractionation of N2H+, we suggest that (15)N(14)N, or (15)N
in some other molecular form, is preferentially depleted onto dust grains.
