\documentstyle{preprint}
\begin{document}
\title{ARE HIGH POLARIZATION QUASAR AND\\
~\\
BL LACERTAE OBJECTS REALLY DIFFERENT?\\
~\\
A STUDY OF THE OPTICAL SPECTRAL PROPERTIES\thanks{Based on observations 
collected at the European Southern Observatory La~Silla, Chile.}}
\author{Riccardo Scarpa\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218\\
\\
E-mail scarpa@stsci.edu\\
\and
Renato Falomo\\
\\
Osservatorio Astronomico di Padova\\
\\
v.\ Osservatorio 5\\
\\
35122, Padova, Italy\\
\\
E-mail falomo@astrpd.pd.astro.it}
\pub{Astronomy \& Astrophysics}
\recacc{December 1996}{February 1997}
\maketitle

\begin{abstract}
The continuum and emisssion llines properties of a sample of 73~blazars 
is studied, investigating differences and similarities among normal 
low-polarization quasars (LPQ), radio-loud high polarized quasars 
(HPQ), and BL~Lacertae objects (BLL).

We found that at variance with LPQ, most HPQ have power law optical 
continua ($f_{\nu}\propto\nu^{\alpha}$) with no indication of blue 
bumps or strong FeII and Balmer emission. Comparison of LPQ with 
blazars, considered as a class including both HPQ and BLL, confirms 
and puts on firm statistical bases that the optical continuum of 
blazars ($\alpha_o=-1.00\pm0.09$) is significantly steeper than that 
of LPQ ($\alpha_o\sim-0.5$). On the other hand the average spectral 
index for the HPQ and BLL subsamples is rather similar 
($<\alpha_{HPQ}>=-1.01\pm0.17$ and $<\alpha_{BLL}>=-0.99\pm0.11$).  
In the $\alpha_o-\alpha_{RX}$ plane HPQ do not split in two distinct 
groups as do BLL, which split in Radio (RBL) and X-ray selected (XBL) 
objects.  All HPQ cluster together in the same region occupied by RBL, 
while XBL are clearly distinguished.

A significant correlation is found between the maximum percentage of 
optical polarization and both $\alpha_o$ and $\alpha_{RX}$, the less 
polarized object having the steeper spectral index.

The comparison of emission line luminosities shows that LPQ have on 
average stronger lines ($\sim$ a factor of 6) with respect to blazars.  
Comparing HPQ and RBL we show they exhibit similar ranges of line 
luminosity, BL~Lacs being just the objects with lines of smaller 
equivalent width as directly induced by the e.w.~$<5$~\AA\ definition 
criteria.

This leads us to conclude that from the point of view of optical 
spectral properties HPQ and RBL are quite similar objects while XBL 
may be different.

\end{abstract}
\end{document}