These, for many people, are the most interesting variables to look at, and they are also right at the forefront of modern astronomical science. Their interest to many observers lies in their unpredictability; you can search the same bit of sky for days, sometimes weeks, at a time without seeing anything out of the ordinary, and then All of a sudden, there is a new kid on the block. You have observed a dwarf nova, for these stars seemingly appear out of nothing, just like a real nova. Of course, that isn't really true. What has happened is that a usually faint star has increased dramatically in brightness - which is basically what a nova (dwarf or otherwise) is. The main difference between real novae and the dwarf variety is simply one of scale; while novae erupt by twelve to thirteen magnitudes possibly once every few hundred or thousand years, the dwarf novae outburst on more human timescales of days or weeks.
A great attraction of these stars is their individuality; each has its own quirk, and while there are not many bright representatives (SS Cygni and U Geminorum are two of the brightest) small to moderate telescopes will at least allow you to catch their outbursts. So, here are some of my favourite dwarf novae. It's worth noting that there are two main types; the U Geminorum (UG) and Z Camelopardalis (ZC) stars.
The prototype of the subclass, Z Cam stars are distinguished from the U Geminorum stars by their behaviour. Whereas U Gems undergo periodic outbursts as described above, these are different. They tend to be constantly varying, but these variations are interrupted at unpredictable times by periods of inactivity, called "standstills". These can last for days, weeks, or months. The Z Cam stars have smaller amplitudes than the U Gems - usually no more than 3 magnitudes. Z Cam itself varies between magnitudes 10 and 13-ish, and is probably the best observed of all these stars, as it is easy to find and visible all year round, though from Britain observation is made harder in the Summer months by short nights and relatively low altitude. This section of its light curve shows typical Z Cam type variations, starting with a standstill.
Like all the Z Cam objects, this has a shorter average period than the U Gem stars, here the outbursts come only 11 days apart, so you can even catch it brightening during the course of a few hours. The upshot is that it always seems to be doing something, and since it reaches magnitude 11 at maximum, even small telescope owners can catch it. Note especially the 'supermaximum' around JD 2451225, and the frequent normal outbursts. Several U Gem stars show this trait, when they become even brighter than usual, and for longer as well. The type star of this lot is SU Ursae Majoris, so if you see a star described in the literature as 'UGSU' you will know it is one of these.
For some reason, the small constellation of Cancer seems to contain proportionally more eruptive variables than most others, such as the UG/ZC stars AT, AK, CC, GY, GZ, HH, DE and EG Cnc - but bear in mind that the Sun and Moon visit here, so there are going to be times when your favourite variables will be unobservable.
A U Geminorum star this time, this fellow erupts on average about once a month up to magnitude 12 or sometimes a bit brighter. It lies in the North of this zodiacal constellation, near a most distinctive bright group, and in turn can be readily identified from a nearby tiny cluster of thirteenth-magnitude stars which are excellent for comparison purposes, and there are also a couple of fairly bright galaxies nearby. One of my favourite stars of this type.
Another U Gem star, varying between magnitudes 10.5 and 14.7. Owners of telescopes with apertures larger than 25cm can catch this object at minimum, and maxima come about 50 days apart as a rule. There was a time in the 70's when SS Aur stopped behaving like a regular U Gem star and displayed "standstills" just like the Z Cam stars. All this goes to show that you must never take variable stars for granted - especially dwarf novae! The light curve here illustrates well the irregular timing of the outbursts of these stars; though the gap in observations is due to the Summer holiday this star takes, when it is too low in the sky for effective observation.
This star has only been regarded as a Z Cam object fairly recently, though as it varies between magnitudes 12 and 14, average-sized telescopes will show it, in its beautiful, inky black, milky way field. It needs constant watching as it has been known to spring the occasional surprise! It often (according to my own results, anyway) goes through protracted faint periods for several days at a time, which is atypical of this type of variable. Again - expect the unexpected.
This is one of my favourite variables, mainly because it is so easy to find and estimate. It lies in a pretty group of 12-13m stars not too far from the Beehive cluster, M44, right on the ecliptic, and I remember one year that it was unobservable for a time due to the proximity of Jupiter. SY Cnc is another Z Cam star, and has relatively few standstills, but is more given to continuous variation. As with all ecliptic fields, you should always beware the presence of asteroids - especially before you announce to the world that you've discovered a "nova"!
Everybody likes this one! The reason is the stunning field of brilliant stars it lies in. This little "cluster" is even visible in binoculars, and lies right on the border with Hercules. The variable itself has a short maximum-to-maximum period of only 17 days. You do need a large scope to show it, even at maximum, and when at minimum you really need a bit more than just your eyes to estimate it. This is another one of those fields which appear darker than normal, so you may be able to see quite faint here.