When I am addressing someone and say:
Кать, приходи в гости в воскресенье
Кать just a spoken reduction of
Катя, or is it a separate grammatical form?
The historical Slavic vocative has been lost in Russian, and currently can only be found in certain cases of archaic expressions. Few of those expressions, mostly of religious origin, are very common in colloquial Russian:
Боже! (vocative of
Бог), often also used in expression
Боже мой! ("My God!"), and
Господи! ("Lord"), which can also be expressed as
Господи Иисусе! ("Jesus"), vocative is also used in prayers, e.g.
Отче наш! ("Our Father!"). These expressions are used to express strong emotions (much like English "O my God!"), and are often combined (
Господи, Боже мой).
In modern colloquial Russian given names and a small family of terms often take a special "shortened" form that some linguists consider to be a reemerging vocative case. This form is applied only to given names and nouns that end in
-я, which are optionally dropped in the vocative form:
Лен, где ты? ("Lena, where are you?"). This is basically equivalent to
Лена, где ты?, the only difference being that the former version suggests a positive personal, emotional bond between the speaker and the person being addressed. Names ending in
-я acquire a soft sign in this case:
Оля! ("Olga!"). In addition to given names, this form is often used with words like
папа, which would be respectively "shortened" to
пап. In plural this form is used with words such as
Such usage differs from historical vocative (which would be
Лено in the example above) and is not related to such historical usage.
"Кать", "мам", "пап", etc. are colloquial form of the corresponding words. Grammatically, such form does not exist. Hence, "Кать, приходи в гости" is a spoken reduction of "Катя". Unlike in Ukrainian (endings "-о" or "-у") or Georgian (ending "-ო") languages, Russian does not have a special "addressing" grammatical form.
Possibly this is a way to shorten a phrase to express the thought quicker, without loss or change of meaning.
"Кать", "мам", "пап" и т.д - это оборот разговорной речи. Грамматически, такой оборот не является правильноый формой. То есть, "Кать, приходи в гости" - это именно сокращенный вариант от "Катя". В отличии от, например, украинского (окончание "-о" или "-у" ) или грузинского (оконончание "-ო") языков, в русском языке т.н. звательного падежа нет.
Возможно, таким образом укорачивается фраза, чтобы быстрее выразить мысль без потери или изменения смысла.