A trend in polarization as predicted by theoretical models was validated, and atmospheric dust grain sizes and projected rotational velocities for these objects were estimated. Comprehensive studies of UDs are proving to be crucial not only in our understanding of UDs but also for star and planet formation as brown dwarfs represent their lower and upper mass boundaries, respectively. Brown dwarfs (BD) were mere theoretical astrophysical objects for more than three decades (Kumar (1962)) till their first observational detection in 1995 (Rebolo et al. (1995), Nakajima et al. (1995)). These objects are intermediate in mass between stars and planets. Since their observational discovery these objects have been studied thoroughly and holistically.Various methods for searching and characterizing these objects in different regions of the sky have been put forward and tested with great success. Theoretical models describing their physical, atmospheric and chemical processes and properties have been proposed and have been validated with a large number of observational results. The work presented in this dissertation is a compilation of synoptic studies of ultracool dwarfs(UDs)¹. [Footnote 1:]. [bullet] A search for wide binaries around solar type stars in upper scorpio OB association (Upper Sco) do indicate (the survey is not yet complete) a deficit of BD binaries at these large separations ([less than] 5AU). [bullet] Twenty six new UDs were discovered at low galactic latitudes in our survey from archival data and a novel technique using reduced proper motion. [bullet] Six field UDs were discovered by spectroscopic follow-up of the candidates selected from a deep survey. [bullet] Optical interferometry was used to independently determine the orbit of the companion of HD33636 which was initially determined using Hubble Space Telescope(HST)astrometry and radial velocity found. Some inconsistency in the HST determined orbit and mass. [bullet] Optical linear polarization in UDs was used to investigate the dust propertied in their atmospheres. Footnote 1: We use the term "ultracool dwarfs" as the mass of most of the objects mentioned is unknown, which is required to classify an object as a brown dwarf. We define objects later than M7 as ultra cool dwarfs.

In this thesis I present the search for ultracool dwarf companions to main sequence stars, subgiants and white dwarfs. The ultracool dwarfs identified here are benchmark objects, with known ages and distances. The online data archives, the two micron all sky survey (2MASS) and SuperCOSMOS were searched for ultracool companions to white dwarfs, where one M9 1 companion to a DA white dwarf is spectroscopically confirmed as the widest separated system of its kind known to date. The age of the M9 1 is constrained to a minium age of 1.94Gyrs, based on the estimated age of the white dwarf from a spectroscopically derived Teff and log g and an initial-final mass relation. This search was extended using the next generation surveys, the sloan digital sky survey (SDSS) and the UK infrared deep sky survey (UKIDSS), where potential white dwarf + ultracool dwarf binary systems from this search are presented. A handful of these candidate systems were followed-up with second epoch near infrared (NIR) imaging. A new white dwarf with a spectroscopic M4 companion and a possible wide tertiary ultracool component is here confirmed. Also undertaken was a pilot imaging survey in the NIR, to search for ultracool companions to subgiants in the southern hemisphere using the Anglo-Australian telescope. The candidates from that search, as well as the subsequent follow-up of systems through second epoch NIR/optical imaging and methane imaging are presented. No systems are confirmed from the current data but a number of good candidates remain to be followed-up and look encouraging. A search for widely separated ultracool objects selected from 2MASS as companions to Hipparcos main-sequence stars was also undertaken. 16 candidate systems were revealed, five of which had been previously identified and two new L0 2 companions are here confirmed, as companions to the F5V spectroscopic system HD120005 and the M dwarf GD 605. The properties of HD120005C were calculated using the DUSTY and COND models from the Lyon group, and the age of the systems were inferred from the primary members. For GD 605B no age constraint could be placed due to the lack of information available about the primary, but HD120005C has an estimated age of 2-4Gyr. In the final part of this thesis I investigate correlations with NIR broadband colours (J - H, H - K and J - K) with respect to properties, Teff , log g and [Fe/H] for the benchmark ultracool dwarfs, both confirmed from the searches undertaken in this work and those available from the literature. This resulted in an observed correlation with NIR colour and Teff, which is presented here. I find no correlation however with NIR colours and log g or [Fe/H], due in part to a lack of suitable benchmarks. I show that despite the current lack of good benchmark objects, this work has the potential to allow UCD properties to be measured from observable characteristics, and suggest that expanding this study should reveal many more benchmarks where true correlation between properties and observables can be better investigated.

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