Jaroslav Merc

Symbiotic stars are strongly interacting binaries, which consist of a cool giant of spectral type M (less commonly K) and a hot compact star, mostly a white dwarf (see e.g. the review by Mikołajewska, 2012). Their orbital periods range from hundreds to thousands of days. The mass transfer takes place via the Roche-lobe overflow or via the stellar wind of the cool giant, which is also the source of the dense circumbinary envelope of these systems. Symbiotic binaries are unique astrophysical laboratories in the study of the stellar evolution, mass transfer and accretion processes, stellar winds, jets, dust formation, or thermonuclear outbursts.
 
Depending on the activity observed in symbiotic binaries, they can be divided into two main classes (e.g. Mikołajewska, 2007): classical symbiotic stars (Z And-type) for which an alternation of quiescent and active stages accompanied by outbursts on time scales of months to years is typical, and symbiotic novae consisting of slow novae whose outbursts last for several years or decades and recurrent symbiotic novae showing very short (few days) and prominent outbursts. In addition to the outburst activity, the light curves of symbiotic stars show various other changes related to the orbital motion and intrinsic variability of both components (e.g. Munari, 2019).
 
Based on recent progress in the study of symbiotic stars, we have decided to prepare a new database of these interacting binaries (Merc et al., 2019). The purpose of the database is not only to serve as a catalogue of data (in format of tables for online and offline use), but we have also prepared a web-portal for easy access to this information. For the catalogued symbiotic stars, we have prepared specific object pages with information, references, notes and links. Making the database online allows us to add new objects as soon as they are discovered and update data when new information becomes available.
 
The database is divided into two main parts according to the location of symbiotic variables. The current version of the New Online Database of Symbiotic Variables consists of 276 confirmed galactic symbiotics and another 132 objects suspected of having symbiotic nature. Although the number increased significantly in the last two decades, it is still well below the estimated numbers for the Milky Way (e.g. Lü et al., 2006).
 
The second part of the database consists of 74 confirmed and 84 suspected extragalactic symbiotic systems which are located in 14 galaxies (LMC, SMC, Draco Dwarf, IC 10, M31, M33, M81, M87, NGC 55, NGC 185, NGC 205, NGC 300, NGC 2403, NGC 6822).
 
Many of the known symbiotic stars are only poorly studied. Ongoing proper characterisation of both symbiotic components in a number of symbiotic stars will allow a better understanding of the whole population. At the same time, the analysis of the candidates is needed in order to provide a clean sample of symbiotic stars in the Milky Way.
 
The number of extragalactic symbiotic stars and candidates have increased by the factor of 9 since the beginning of this century (Merc et al., 2019). The greatest advantage of studying extragalactic objects is that their distances are usually known with sufficient precision. The parameters of the components could be determined more precisely, which is very useful for comparing observational features with theoretical models.

Our catalogue can serve as a basis for the systematic studies of the symbiotic population in the Milky Way and in the galaxies of the Local Group, where the objects evolved in the environment with different chemical composition. The database could also be useful as an input for modern approaches of searching for symbiotics, e.g. using machine learning.