  Low-dimensional organic conductors show a rich phase diagram, which has,
despite all efforts, still some unexploed regions. Charge ordered phases
present in many compounds of the (TMTTF)$_2 X$ family are typically studied
with their unique electronic properties in mind. An influence on the spin
arrangement is, however, not expected at first glance. Here, we report
temperature and angle dependent electron spin resonance (ESR) measurements on
the quasi one-dimensional organic conductor (TMTTF)$_2$NO$_3$. We found that
the (TMTTF)$_2$NO$_3$ compound develops a peculiar anisotropy with a doubled
periodicity ($ab'$-plane) of the ESR linewidth below about $T_\text{CO}=250\pm
10$ K. This behavior is similar to observations in the related compounds
(TMTTF)$_2 X$ ($X$ = PF$_6$, SbF$_6$ and AsF$_6$), where it has been attributed
to relaxation processes of magnetically inequivalent sites in the
charge-ordered state. For the structural analogous (TMTTF)$_2$ClO$_4$, known
for the absence of charge order, such angular dependence of the ESR signal is
not observed. Therefore, our ESR measurements lead us to conclude that a
charge-order phase is stabilized in the title compound below $T_\text{CO}
\approx 250$ K.
