  We report the discovery of a previously uncatalogued arch-shaped filamentary
nebula at the outer part of the Triangulum galaxy (M33) centred at R.A. =
1h34m25s, Dec = +30d20m17s (ICRS). This discovery stems from meticulous
observations employing deep exposures of M33, using both H-alpha and [OIII]
narrow-band filters. The nebula, designated as "Roig1 Prades Sky", exhibits an
H-alpha surface brightness of 23.9 mag/arcsec2. Its sky projected location is
21 arcmin away from the M33 galactic centre towards the southeast direction
with an extent of 120 by 440 pc. Deep spectroscopic observations are required
to unveil its real nature.
