  Context. Transition disks (TDs) are circumstellar disks with inner regions
highly depleted in dust. TDs are observed in a small fraction of disk-bearing
objects at ages of 1-10 Myr. They are important laboratories to study
evolutionary effects in disks, from photoevaporation to planet-disk
interactions.
  Aims. We report the discovery of a large inner dust-empty region in the disk
around the very low mass star CIDA 1 (M$_{\star} \sim 0.1-0.2$ M$_{\odot}$).
  Methods. We used ALMA continuum observations at 887$\mu$m, which provide a
spatial resolution of $0."21\times0."12$ ($\sim$15$\times$8 au in radius at 140
pc).
  Results. The data show a dusty ring with a clear cavity of radius $\sim$20
au, the typical characteristic of a TD. The emission in the ring is well
described by a narrow Gaussian profile. The dust mass in the disk is $\sim$17
M$_{\oplus}$. CIDA 1 is one of the lowest mass stars with a clearly detected
millimeter cavity. When compared to objects of similar stellar mass, it has a
relatively massive dusty disk (less than $\sim5$% of Taurus Class II disks in
Taurus have a ratio of $M_{\rm{disk}}/M_{\star}$ larger than CIDA 1) and a very
high mass accretion rate (CIDA 1 is a disk with one of the lowest values of
$M_{\rm{disk}}/\dot M$ ever observed). In light of these unusual parameters, we
discuss a number of possible mechanisms that can be responsible for the
formation of the dust cavity (e.g., photoevaporation, dead zones, embedded
planets, close binary). We find that an embedded planet of a Saturn mass or a
close binary are the most likely possibilities.
