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Photometric and spectroscopic observations of five nearly parabolic cometswith eccentricity larger than 0.99 at heliocentric distances greater than 4 AUwere performed. No molecular emission was observed for any studied comet andthe entire cometary activity in all cases was attributed to dust production.Upper limits of the gas production rates for the main neutral molecules in thecometary comae were calculated. The derived values of dust apparent magnitudeswere used to estimate the upper limit of the geometric cross-section ofcometary nuclei (upper limits of radii range from 2 km to 28 km). Due to thepoor sublimation of water ice at these distances from the Sun, other mechanismstriggering activity in comets are discussed.

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Observations of Comets C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3  (WISE), C/2010 S1 (LINEAR), and C/2012 K6 (McNaught) at large heliocentric  distances